Go to the library to borrow CDs. I find that I'm much more willing to experiment with new artists and styles when I'm not paying by the pound just to hear them. If you're especially resistant to new music, then give yourself a quota (5 out of 10 CDs have to be from outside your usual genres).
It can be a little frustrating to find that many discs are scratched, but if you give yourself plenty of time and look around in all the bins, you'll probably end up wanting to take more albums than you're allowed.
Sure you'll end up borrowing plenty of stuff you don't like, but what've you lost? Also, give music more than one chance. Often I don't like something the first time I hear it, but it might just not be right for my mood at the moment.
And I suppose a few reminders can't hurt:
1) Take great care with library CDs. If you've ever browsed through the bins at a public library, you know what I mean. Some people must use these discs to play frisbee with their dog.
2) Buy albums that you really like. Especially if they're not by huge/famous groups. Don't just "burn & return" (hey, that should be a bumper sticker or something...)
3) Drop a couple of bucks into the donation box every once in a while, to pay for new materials (and to replace damaged ones). Or do what I do and pay through the nose for late fees! My library uses these fees for materials also. Seriously, I should have a wing named after me at this point.
For myself, I second what someone said earlier -- I go in cycles. At some times I just don't listen to much music at all, then I'll go through a phase for enjoying a certain genre, artist or composer, then on to something else. Phases can last from a couple of days to several months. I consider myself to have extremely diverse taste in music, but I don't usually plumb the extremes simultaneously.
