On long journeys we always forget to track supplies so I am looking for a way to make supplies actually matter in D&D 3.5.
The starting point should probably be to take efforts to not forget to track supplies. To help with that, you could, say, print out/write down the Starvation and Thirst rules on a note and stick it to your DM screen.
That being said, supplies generally don't matter in 3.5. There are just too many ways to trivialize it, even if rigorously tracked. Here are some reasons:
If you wanted to make rations matter, you'd have to start cutting these down, one by one. And I wouldn't recommend it for most games - it's all too likely to end closer to "ration accounting 101" than the "gritty survival horror" houserules like this tend to be aiming for. You may want to check out a different system if that's what you're after.
You could try to have the rations give a small bonus. If there is a boost involved, the players are more likely to be interested.
As an example: Divide food into three categories: Hearty, Energizing and Comforting. If the character eats hearty food, he gets a +1 Fortitude bonus. Energizing food gives +1 Reflex, and Comforting gives +1 Will.
Then each morning the players need to state what kind of food they are eating, and scratch it off their supplies.
Add flavor; hearty food might be venison or good iron rations. Energizing food might be fruit or might simply be adding coffee to standard rations. Comforting foods might be candy, or a lunch packed by the inn-keeper's daughter.
Add complications; Maybe energizing food is ruined if it becomes wet. Maybe you lose Will for a day or two after having eaten the last of the comforting food.
I had to take care of something similar for an adventure I ran once (it was for Silhouette, not any flavour of D&D, but the principles still apply, I believe).
The PCs were "under siege" in a sort of barrack/tool shed close to a mining entrance. So they had a selection of stuff they could take with themselves, and it was clear they had to try to hide in the tunnels to escape their enemies. This made the logistics (do I take rope or 4 extra rations?) important, because they had not much time, the item selection was limited, but still they couldn't take all with them, and they had no idea if they would need more food, more water, more rope...
Basically I created (designed in Word and printed with my PC) small "character sheets" for the items that they had at their disposal. In case of small items that could be taken together, I created sheets with tick boxes a bit like this:
If someone wanted to carry say, 20 rations, they would just cross off the extra ticks. (And of course I asked them to cross one off whenever someone ate a ration).
For a lantern, for example I'd do something like:
The system I was using wasn't super-fussy about encumbrance, so I winged it (let's say you have 10xSTR+5 carrying capacity - Silhouette stats are 0-based, -5 for your weapons and clothes... Rations "weight" 1/5 of enc unit, lantern is 2/5, 100 yards of rope is 2 units etc.)
This way the players could discuss and take decisions, the items had a token that represented them in play, and consumable could easily be crossed out (the mines were difficult terrain, so in some places you had roll vs. Athletics or lose some random item, like 1d3 rations etc.)
I like the other answers, but an alternative approach is to focus on logistics and role playing.
The bottom line, as others have pointed out, is that minute bookkeeping is very rarely fun. A logistical challenge can be interesting though.
I should say, I have never made food a major mechanical issue in any game. But it can be a good flavor issue and a major logistical challenge can be interesting (I haven't done this in 3.5, so it might just be too trivial there).
In a game where supplies are meant to be an issue, make sure the party has a supply plan in place. I don't track every bit of food and I rarely make it a mechanical thing at all, but it makes them think a bit about the combination of magic, foraging, and carried supplies they will use. I then mention (but don't keep track of) of what they found, their use of magic. When things are lean, I mention the hunger pains, the way lack of supplies gets on nerves (especially if there are NPCs with them). A successful hunter or forager could easily be the hero of the day. A significant source of rations might be a major plot point, even if it isn't tracked mechanically.
But of course, all of that is only interesting if there is something making it challenging. In a post-apocalyptic world where food is scarce and something another group will fight you for, then its interesting. If they have magics that let them generate food or sustenance they might easily get attacked for that. If they managed to get a rare large animal, they might have to fight with other two legged scavengers to be able to keep it. Even in a campaign that isn't survival driven, it can be interesting for them to need to carefully defend that donkey that is carrying their provisions when they get ttacked. But its interesting as plot points, background, and a conflict driver, not in fine bookkeeping with mechanical effects.
The other way its interesting is if you have a truly large group, like say a large army unit, or your enemy does. Many wars in the real world have been won or lost based on logistics. Sure scavenging for a small group is easy in many environments, and a few magical items or spells to sustain a small group might be trivial. But if you have an army you need to worry about keeping and maintaining your logistical line. Scavenging for such a group is on the lines of seizing a hostile grain store, not hunting a brace of rabbits. That again can drive plots if you have to make tactical decisions to protect a long supply line or moral decisions about letting the local civilians starve so you can feed your troops. A potent magical cauldron capable of feeding a platoon might just be a macguffin to get your players to try for a stealth mission to try to seize it to help with the supply situation.
Make them rare.
In most settings, it is incredibly trivial to get the sort of supplies you need to survive in the wild. Rations are incredibly cheap. Pack animals are reasonably priced. Magical items are available that make sustained food consumption even more trivial. Some races don't need food, and in most cases a survival check to eat is not difficult.
But there are no rules saying you have to provide any of these things to the player.
In an ongoing campaign I am playing, our DM made it perfectly clear that survival rests entirely on our shoulders, and so far it has been the major threat of the campaign. I and the other player are essentially Norsemen on a lone quest. We are allowed the clothes on our back and a small number of goods for survival provided by our tribe.
There is no 'merchant' from which we can buy rations. It is extremely cold, and blankets and fire are essential. Harsh weather and difficult terrain adversely affect our ability to hunt and forage for food. Most importantly, we are not elves and we have absolutely no magic.
We also have to hunt if we want meat, not 'make a survival check' (that only helps us find an animal we could eat), meaning the other party member has to use up arrows to shoot game, while I have to basically attack them with my sword. And most of the time we do not have much luck, barely scraping by with what we find.
We still have fierce encounters of course. Like fighting off packs of wolves on ice packs, or horrid sea beasts that burst forth from the ice we are trying to cross back over. Oh, and undead warlords from past tribes. Yet the constant threat we face is freezing to death or starving each day.
This doesn't have to be done in a brutal tribal setting either. "Trail Rations" may be listed under equipment, but there's no rule saying you have to make a merchant sell those rations at every town they come across. Pack animals may be cheap-as-listed, but if it's the only mule for miles, double or triple the price to make it a real luxury. Ban elves if you really want to make them work for their supper (Unless you have a player who really loves elves, this shouldn't be a problem). Limit the party to one druid or ranger at most, if that's going to cause problems.
And feel free add a few modifiers to make foraging more difficult (poor crops, blighted lands, unfavorable weather, unfavorable terrain, being unfamiliar with the land they are in, just to name a few).
Food and transport aren't the only things that you need to survive either. Consider, for example, shelter and comfort. Can you really honestly say that your heroes are going to be well-rested after sleeping a night on the ground, in full armor, in the middle of the winter? They would freeze to death, or at the very least be exhausted or fatigued(conditions you can enforce!) from the lack of sleep. Make them take off their armor and put away their weapons if they want to make camp. Make it dangerous to start a camp without a fire, for fear of the wolves that will attack if they let it go out (and make them sleep in shifts to re-enforce this fear).
There may not be any rules against sleeping on the ground in Core, but there are rules for effects of extreme environments, including cold temperatures such as ice-cold nights. So make them suffer those effects if they fail to bring a bedroll or at the very least a blanket. And encourage them to bring a tent for shelter against the rain (because of course you should make it rain once in awhile).
In short, if you want to make equipment non-trivial, make it hard for your PCs to actually get equipment. If you want to make hunger and other environmental conditions non-trivial, make your players fight these conditions constantly, and make it clear that they will suffer if they do so poorly.
Of course, given that these matters are trivialized in many games, you should be sure to make it clear from the start that equipment will not be trivialized, or else all you will be doing is enforcing a -2 penalty onto everyone without any sort of understanding for the player as to why this is happening.
Hiking with a pack of 35lbs would at a good pace would consume 7000 calories for a male 160lbs. A larger male would need 9000 calories. Long distance hikers consume whole cartons of ice cream and bottles of olive oil.
Adventures aren't long traditional long distance hikers. First off they are carrying 50lbs of weapons, armor, and adventuring equipment. Their packs would be double the average hiker at least. Also, fighters are "always" in their armor. Armor isn't comfort or easy to move in. It would take effort. Effort requires additional calories.
All of this calorie expenditure would dramatically affect con, str, and dex, not to mention int, wis, and cha as delirium sets in. Adventurers wouldn't be able to eat enough calories. Starvation, illness, and accidents would actually be how most adventurers would die.
With this bit of realism in mind, I think supplies stop being trivial. It makes it another form of survival.
Always remember that RPGs are only interesting when you get to make decisions
Accounting and bookkeeping aren’t interesting because usually they’re less “do I take this or that?” and more “well now, have I forgotten anything?” Even “do I take this or that?” isn’t very interesting if you have no information to go on: if it’s just a toss-up which you take, getting it right doesn’t feel like success and getting it wrong is bad luck rather than a mistake. That’s not interesting.
Therefore, don’t leave supply issues too open-ended. This causes problems of almost “reverse-metagaming,” where the player doesn’t know (or remember) things that the character would. Make sure you indicate to your players what options they have (in a general way and of course leave them opportunities to be clever). If supplies are short, make it clear what options would be obvious to them, and what consequences they can reasonably expect for doing so.
I left out the Cleric because, well, the Cleric would just be:
I think the easiest thing to do to reduce bookkeeping and make it simpler is to (instead of treating each piece of food as something else to keep track of) group rations together. Figure out how much your group needs to eat each day (per person) and how much they need together. Then figure out how much enough food for one day for everyone should cost and how much it weighs.
Example: Let's say each human or human-like creature needs ~2500-3500 calories (They are walking/fighting all day, which tends to make one hungry). That's several loaves of bread, or maybe one loaf, a half-pound of red meat (dried/smoked), some vegetables and a flagon of beer or two (that's the stuff to give the troops!). Feel free to tweak the numbers if you disagree. All together, I'd guess maybe 2-4lbs, so let's say 3lbs per person. I'd estimate the price at 5sp, although it depends what the economy is like in your game, and what different things cost. If the PCs want to pay a little more for more compact food, raise the price to 1gp or so.
Thus, if we have a party of 4 (human, half-elf and two dwarves) who can be expected to eat about the same, one day of food for everyone weighs about 12lbs and costs 2gp. If they are going to be travelling for a week, that's (7 x 12 =) 84lbs and (7 x 2 =) 14gp. However, after 2 days they decide to turn back. After 4 days' travel, they're back home. Take off (4 x 12 =) 48 lbs of weight to leave 36 lbs of food (12 days of food for one person, or three days for four people).
The advantage of this is that if the group is exploring a dungeon, they can pay for food in days at a time, letting them plan ahead. Also, they and you know when the food should be gone. If you have someone Creating food, then they simply produce their own food-days, which you can add to their total.
Optionally: To simulate spoilage, make food keep for about a week (depending on what it is - if the group pay more, it should last longer) then roll 1d6 for each food-day left. If a 1 comes up, it's spoiled. Increase/decrease the numbers if you want.
This system saves bookkeeping because once you've worked out the price and weight of food-days, you only have a single, small number to keep track of.