In a nutshell, when they do something that makes me think "Whew! They've really earned that next level!" (achieve a major goal, solve a big mystery, overcome an obstacle that leads to the next big thing, make it through a deadly or otherwise very dangerous encounter, etc.), that's usually when I say, at the end of the session, "level up for next week, guys!", and all the players go "woohoo!" or the like.
Maybe that's super-obvious, or maybe that doesn't answer your question. It's a reward for doing the really fiddicult thing, and a reward that they know is coming if they can actually get that really difficult thing done.