I admit that I'm usually all about taking advantage of your instincts to hose your players to make things more fun for them, but I want to take a second to turn that around.
See, while I've gotten better, one of my real weaknesses as a GM is a tendency to be too nice. If things fall apart or go horribly wrong, my instinct is to step in with my GM authority and help save the day. This is a terrible habit - I just can't stress it enough. Even if I'm not setting forth to show the players the story of my cool NPCs (which I'm not), having them step in to dramatically save the day has the same net result. My game's no longer about the players, it's now about whatever the stakes of this particular adventure are.
This is a sinister problem, in part because it comes from a well-intentioned place. Your players are maybe upset and disappointed with the way things have gone, and you want to mitigate that. It's totally human and understandable, but it will suck the fun out of your game.
However, like all bad habits, advice to "Just stop doing it" is basically useless. There are reasons people have the patterns they do, no matter how "obvious" things may seem to people outside those patterns. So if you have this instinct and you want to change it, then the trick is not to stop, but rather to redirect it.
So, the net time you find yourself in this situation, stop for a second as you pull it all together. Things have gone badly, so you've got the cavalry ready to ride. This is the point when you should stop and think - _how_ is the cavalry going to save the day? You need to have a better answer for this than "With their sheer awesomeness" and you probably will have one, because hey, you're a good, thoughtful, conscientious GM and even if you're helping you're not just going to pull an Elminster.
Once you've thought about that, think of it as a plot seed. Specifically, think of it as something that's ready to go but is missing one key element. Then make that element the player's responsibility.
This may sound tricky, but it's surprisingly easy, and it's something you see in fiction all the time. Consider the scenario where the cavalry literally shows up to help - they've got the men, they can win the battle, but they're pinned down by the artillery up on that mesa. Clearly, someone needs to sneak up there and take out that artillery team! Really, look at almost any fiction where the backdrop involves huge, powerful forces (like a war) and you'll find eamples of how the story narrows down to some lynchpin action on the part of the protagonists.
And now here's the real dirty trick - once you've gotten the hang of doing this, it becomes a trick you can incorporate into all your adventures. This is especially true if you want games against a big backdrop, or ones with powerful NPCs calling the shots, like The Forgotten Realms or any of the older World of Darkness products. If it's important to you that things and people be bigger than your PCs, then you can still keep things robust by getting the movers and shakers up to 90% but have them need help to get that last 10%.
This works in most play models, even classic mission-based play, but it has the advantage of giving the missions a reason that is somewhat more significant than "The Prince can't be bothered. You go do it". And more, by given the players even a small part in big events, you'll find that it ties them into events more tightly over time. These events, after all, are the things that NPCs respond to, and if players have a tie to the event, that's a one-step-removed tie to most of the interesting NPCs.
Plus as a bonus, it makes something that's historically a drag into a real play booster. Normally, the more invested you are in what Khelben Blackstaff or the Malkavian Antediluvian are up to, the less invested you end up being in your players, but by looking for that 10%, that lynchpin, you turne that investment back onto the players, hopefully to a good end.
Anyway, I try not to be a nice GM these days, but the habits are still there. For me, it's useful to have a practical way to channel them.