In recent years, many games have used this simple and consistent approach to fixing the problems you describe.
If you don't want characters to die, take death off the table mechanically! I mean, that's just basic logic, really: outcomes you know you categorically don't want aren't worth including in the set of possible consequences.
Emphasize goals, not battles of pure attrition. Characters are fighting their enemies for a reason, right? Focus combat scenes around those reasons. When the opposition knows their cause is lost, have them melt away and abandon the field. When the PCs' aims have been frustrated, let them know and transition to a different kind of scene. Bankuei's Big List of Combat Stakes is about as good a place to start as any.
As a bonus, shifting to goal-oriented combat allows for a game structure where the battles are more than just a foregone conclusion. The PCs can lose a lot without ruining the whole game. This means you can actually present fights that are more tactically challenging for the players (if they enjoy that kind of thing).
The best way to avoid "Oh, whatever, the GM will save us!" is to trust each other, communicate clearly, and follow a consistent set of GM principles. Don't worry about the players knowing what the principles are. That's good! It means they can play more assertively since they know they can trust you to "back them up" a bit.
I mean, if you can't trust the other players to portray their characters with good faith, honesty, and fidelity to the dramatic situation, why are you playing a character-driven game with them in the first place?
Fixing D&D/Pathfinder (Partially)
Unfortunately, the simple answers aren't necessarily the easy answers, in this case. You're working with a huge, mechanically complicated game that doesn't support any of this stuff out of the box.
A particularly thorough fix is simply more trouble than its worth. If the "simple answers" truly speak to you, look into a system designed with these ideas from the ground up.
Here are some "quick fixes" you can use with D&D3.x/Pathfinder, though:
Switch to goal-based combat. This'll create much more fertile ground for character-driven play in combat (which, let's be honest, takes up a lot of a D&D/PF game's session time). This'll also reduce the likelihood of character death by shifting the focus away from battles of attrition.
In my experience with D&D, a cool giant "set piece" battle is much more fun than the four CR-appropriate encounters the book suggests. Goal-based combat lets you actually play that "set piece" as an interesting tactical and dramatic situation, with even more potential to interact with the NPCs and scenery in new and novel ways.
If a fight has no larger purpose, it's filler. Learn to recognize filler and cut it.
Tweak the death rules to match your goals. As written, there's a very narrow window between "down" and "dying." The bleeding rules generally encourage other characters to get themselves into trouble for fairly little gain, too. Some good options:
Create a more expansive "knocked out" state, so that it takes a lot of damage to truly kill a PC. For example, anything between 0 hp and -(20+level) hp is "knocked out" — you're out of the battle but won't die unless more bad stuff happens to you. To keep some of the sting in being knocked out, you can make it harder to revive a downed character, or impose a condition (try fatigued/exhausted or temporary energy drain).
As above, but don't use negative hit point bands at all. Instead, just have states: anything that could kill you just renders you "knocked out," and then there's a rather involved mechanic for actually dying. If you fail your save vs. Disintegrate, you're in that knocked out state, not actually dead yet.
Replace death with other kinds of lasting bad stuff. When the rules normally say you'd die, instead you get some kind of debilitating injury. You can get pretty creative with these, and also make them thematically appropriate to the kinds of crazy magical adventures the PCs are having.
Keep the death rules but give characters a limited supply of "fate-dodging" points. Lots of games have a mechanic you could borrow.
This answer has some additional information about how to hack D&D to be less lethal (especially less randomly lethal) without making it bloodless altogether.
Tweak the reward systems to match your goals. A great way to get extra focus on combat goals is to reinforce them with rewards. Hand out XP for following your motivations or general campaign goals (here's an example; here's another) or do away with XP altogether and just have the PCs level together at whatever rate feels appropriate to you all.
Trust your players to work with you. I think this bears repeating. A lot of advice for D&D/Pathfinder assumes that the GM is the boss and the players are just there for the ride. Also it pretty much assumes that players are terrible and you'll need to trick and cajole them to get a good game. Seriously: just don't bother with that noise! Work together to make the game you want. It's like ten times easier.
This is a bit of a side issue, but I think it's important to point this out.
Think long and hard about what "bad decisions" are, and whether you truly want to "punish" players for making them.
For starters, how important is challenge-based play in your game? Are you actually looking to test the players' tactical skills thoroughly?
On top of that, is that critical hit truly a "bad decision?" How 'bout failing a save? Was the player actively tempting fate or just, you know, facing danger like a hero is supposed to? Heck, maybe you want the characters to tempt fate a bit. I mean, by most sane standards, going on adventures is a terrible decision. But it's presumably one that you want the protagonists to keep making consistently.
In character-driven dramatic play, characters making bad decisions is usually... good. They reveal character and push the story in cool unforeseen directions. Bad decisions should have serious fictional consequences — without that, the story is toothless and the "decision" isn't a real one — but they're not something to "punish" at the table.