I've played in a couple of play-by-post games over the last couple of years, and they've all ended more-or-less unsatisfactorily. Usually posting would slowly wind down until we hit a bottleneck of some kind and it stopped entirely. If you have ever run or played in a PBP or PBEM of this kind that survived more than a couple of months: What techniques and measures in the area of system, style, social contract, or otherwise work to make a game like this maintain player engagement and game momentum?
I've run several successful PBPs and several unsuccessful ones. (It's been a very long time since I did PBEM.) This is based on my own experiences only.
Have good infrastructure
I usually set up a phpBB-based forum solely for use with the PBP, rather than using an area of someone else's forum. It makes it much easier for me to administer and tweak.
I set up custom ranks for each player to indicate which character he or she is playing, which makes it easier for the players and myself to keep things straight. I encourage avatar use as well.
Another advantage of having your own forum is that you can easily set up "private" forums, each of which only a given player can see. This is useful for "note passing," keeping track of xp and private conversations, and so on without having to go through a backlog of PMs.
I usually have an out-of-character forum for intros, general discussion, and metagame stuff, and an in-character forum for actual play. I devote a separate thread to each "delve," as I'm currently running an OD&D campaign.
Be active, have active players, and communicate interruptions
The paramount concern in PBP, in my experience, is having active players. I ask for some kind of substantive post at least once a day. Stagnation is doom in the PBP realm — once things slow down, heat death follows. If I can't post for a few days — which happens — I let everyone know, and expect the same.
Start the game with clear direction
One thing I discovered running a City State of the Invincible Overlord PBP a couple years back is that too many options at the beginning of play can lead to decision paralysis as the players try to decide what to do. In a dungeon-type game, I'd recommended starting at the adventure site, getting stuck in, then getting to the roleplaying after the players have the swing of things. In a story-type game, I'd consider starting in media res, like with a chase scene or something that gets the players thinking in-character quickly.
Pace your background exposition
Finally, I'd avoid excessive infodump. It's great for you, as the referee, to know lots about your setting, but I'd keep the early background to just enough to give the players a sense of things and parcel the rest out through play.
I ran a PBeM game for several years, and have done a number of successful and "failed" with others. I've only done free-form this way, though. (I call it "Collaborative Fiction" when talking to non-nerds.)
One of the keys to keep things going is to be continually recruiting. New blood keeps things alive. For a free-form PBeM, I'd suggest at least 6 players minimum; I prefer 8-10 as my ideal. I've also found some sort of public "standard" helps greatly -- "I will post at least once a week" sort of thing. I've tried "Just post when you're able" games, and they tend to grind to a halt in pretty short order as you've described.
You also want to find and nurture the leader-sorts within the group -- you want people who will kick the group when it slows down too much, either by throwing in new ideas or plot twists or whatever else. You also need to be willing to cut a plot/storyline/quest closed when things start to flag, and start up a fresh new one again -- in my experiences, about a month and a half is enough for a single storyline, which means if you throw in two weeks of down time between plots, you have a nice rough two-month schedule for plots.
As you note, PbP games have a tendency to peter out within a few months, but some great ones last and there are definitely things you can do to maximize their chances:
For the GM
One of the biggest factors will be recruiting great, reliable players. Assuming you don't already know them,
Once the game is running,
For Game Seekers
I recommend looking for many of the same things above, e.g.
I had a PBP game that went on for a few years. Best advice I can give is be very selective of who joins. All it takes is a few people to go stagnant posting late which drags the game on and people get bored. People you know that will post regularly and can commit is the only way to go. If you have people that are not posting per your stated guidelines. Boot them and replace them right away. Otherwise it's not worth your time or your friends. If you treat it like an exclusive PBP, it's more likely to be treated like an exclusive pbp. For the game I had going on for a few years. I was lucky! Great bunch of guys, but after a year or two people get really busy with real life. So try to set short stories so that if someone has to bail, they might be able to at least finish out the chapter in your on going pbp campaign.
The biggest killer I've seen to online play for RPGs is combat with initiative order. So in terms of system, picking one that de-emphasises combat, or at least handles it equally would be necessary. Alternatively, some systems, like Burning Wheel have a combat system that isn't done step by step like most traditional systems, that lends itself better to PBP/E. Another thing is avoiding a system that requires checking against TNs. If you ahve to ask for permission before you declare your action, it really slows things down, but if you can just roll and determine the results yourself, you can string together several actions in a single post. This means d20 systems are a particularly bad fit, because they offend on both counts. Something like Alternity, or even moreso Donjon would work much better.
Secrets to running a play by post game? I would sat the first is having a game YOU LOVE. (If you do not love the theme, world, and such, the game will not thrive).
Save deeper stuff for other things.
1) Don't rush things
It can be tempting to try and start the game off with a flurry of activity, refreshing the thread constantly, replying to every little thing as soon as possible, pushing players to post as often as possible, but ultimately you won't be able to keep this up. The game will start to slow down, and once that starts, there's no stopping it.
The key to a long-lived Play-by-Post game is to embrace the slow, steady nature of the format. Go for quality of posting rather than quantity. If you can, try to post around the same time of day each day, or perhaps one or more of a handful of times. Make it part of your daily routine, checking in once before work, or just before dinner, or after putting the kids to bed, or whatever works for you. If you keep a steady pace, your players will be able to adapt to that pace and work the game into their own schedules, as well.
2) Keep the action dense.
This doesn't necessarily mean combat, but generally if you're waiting for your players to do something, make sure that what you're waiting for them to do is actually interesting. Some decision for them to make, some event or dialogue for them to react to. If you can keep things moving by simply rolling a few dice and/or continuing to describe the scene, do so.
The exception to this comes when you want to give your players some time to chat and interact among themselves, but try to be clear that this is what you're expecting to happen, and if things start to slow down, be ready to jump back into the action again. Ultimately you are responsible for driving the game forwards; you can't rely on your players to move things along for you.
3) Make sure things are clear for your players.
Don't wait for your players to ask about their surroundings, or if they can take a certain course of action. Try to predict what your players will ask or attempt to do, and describe the scene and possible courses of action as best you can. If you think your players might try to ford a river, tell them what they would need to roll, and how high. If you think your players might want to know what weapons the orcs are wielding, tell them. If you think your players might look for possible escape routes, list them.
The goal here is to make sure that your players are posting actions, not questions. This helps keep your players interested; there's always something for them to do when they check in each day. However, this also extends to making sure your players have clear actions to take. They aren't left wondering what they can do, they know what they can do, and how to do it. Of course, it's always possible that your players will take an action that you didn't foresee, and that's half of what RPGs are about, but even then, the more information the player has to work with, the more they'll be able to build on that and come up with something else that you didn't expect.
I have played in and run several PBeM games and as with the other posters, some have failed and some have not. One (sodiumnoir.com) ran for 10 years and it was eventually my failings that killed it in the end. I couldn't handle a disruptive but outwardly helpful player and they eventually ruined it for everyone. Even the person running the game is susceptable to the many things thta can kill a PBeM.
Here are my tips to add the other great advice when running an online game.
1) be strict with troublesome players. The number one killer of an online game is the guy who spoils it for everybody else. You have to keep a harsh control over disruptive influemces, espechially the ones who will spoil the game for you as the GM.
2) share the workload. If the game gets big, give your star players room to run thier own substories. Ask good players to help newbie players, treat it like a community event. If people offer to develope the website, let them.
3) keep everybody reined in to start with. No matter how great a player seems to be, don't let them stretch the rules of the setting or to create an 'unusual' character. make everybody, even your friends you already trust, prove themselevss first. Once they have shown that they are a benefit to this game and this setting, then you can let the good players do kinky things with their characters or even create seciond characters.
4) have a page for the rules of your game. make sure players know the way you deal with non posting and the like. An unyeilding system make sit much easy to deal with issues.
5) make sure players have access to you or the GM/storyteller. Make sure their access to him/her is structured and they don't bug the GM so much he starts to hate the game.
6) the number 2 killer is the GM/storyteller loosing interest. How you prevent this I have no idea. The best games I played in were run in chapters, with natural ends to each story. Even when the GM lost interest and the story fizzled out, they'd still run good stories in the past and that is what I remembered.
7) don't try to fix a lull in the game by throwing new stuff at it. Games can get drown in all the great ideas at times. Every player is a frustrated storyteller of some type and they will all want to throw cool subplots for their character at you. You will want to throw a bunch of cool things at them. Find a way of limiting that and you'll be a better man than I.
I haven't run very many games on PBP myself, but I have been in a few very successful ones. Here's what I've seen that works, specifically for using someone else's PBP site.
Separate OOC and IC
This is absolutely essential in a PBP game. If you're using an RPG forum, keep two threads going at once - one for OOC and one for IC. If the forum has a dice-roller, always roll in the OOC thread, or if you prefer and the forum has it, use Spoiler tags to keep the roll separate from in-game action. If you need to tell a player something secret, send a private message. If you need to discuss actions out-of-character, use the OOC forum OR if you have a small group you could even host an IM chat for more elaborate OOC discussions.
Have Everybody Read The Rules
PBP is one system where not knowing the rules (or not having access to them) can really grind a game to a halt. Make sure your players know the rules before getting started. They don't need to be experts - they just need to know enough to play their character, or to be able to look up a rule if needed. If one player doesn't have access to the rules for any reason, make sure they have access to someone who does, or create a short 'cheat sheet' of things that player will need to know in advance.
Keep People Posting
There will be periods when players aren't posting, and it can be a real drag on the game. Make a minimum post requirement - at least once a week for larger groups, or once a day for smaller groups. Make it clear that if a player needs to go on vacation, they must inform the rest of the group well in advance, and preferably leave a way for the other players to continue.
Be Prepared For A Long Haul
If you're doing a PBP right, it will last for a long time. Not 'might', it will, inevitably, last for a very, very, very long time. Really. A single encounter in whatever system you're using can take weeks to complete. A single quest can take months. And a whole campaign can easily take years to finish. This is inevitable. Accept it, embrace it, and be prepared to take copious character notes in case you forget something that happened literally a year ago.