I rely on two things:
Lots of uber-cool bad guys. Come on, who doesn't like coming up with bad guys? Crazy princesses with a sadistic streak, necromancers who need fresh souls, ambitious viziers and foul demonologists, bad guys are good. Er. Anyway, have LOTS of bad guys. Lots of really TOUGH bad guys. And keep 'em busy. They don't sit around waiting for heroes to come after them -- they've got things to do, souls to crush, nations to subjugate. If the heroes don't come after them, bad things will happen. And since they're so tough and so busy, they'll have plenty of minions to deal with pesky adventurers, so your PCs will have to hew their way through large sets of mooks -- and by the time they've gotten the first bad guy, the others will start paying a little more attention...
Cool stuff. I know I said your bad guys should be cool, and they should, but in a way they're a subset of the vast amount of cool stuff your campaign should contain. The great thing about cool stuff is it's so easy to generate. Just decide what you think is cool and put it in. Ruined jungle temples? Sure. Flickering rifts in the sky that lead to other worlds? Rock on. Talking penguins? Whatever you like. Stuff your campaign with all the cool stuff you can think of, and then whenever you get tired of what's going on right now, pull out your list of cool stuff and bingo, you've got half-a-dozen adventure ideas.