First, the basics of photography should be pretty much mastered. No strong foundation, no good pictures, no matter what.
Once you get that down, you have a god base for getting good pictures. After that, it is a simple (lol) matter of learning how to get to know your subject within a VERY short time (usually a few minutes), and leveraging this information to get the kinds of pictures that bring out their personality.
I'll be doing my first set of seniors later this year and already am working on things like specific questions to ask (don't say "tell me about yourself..." thats the fast question to nowhere... lol). Practice helps, don't go in cold. Grab a couple friends who's kids are seniors and want to get a few pics done and practice, pactice, practice!
Finally, for inspiration, visit flickr and do a search for senior pics. There are tens of thousands of available ideas there ready to be used and abused!