We're still looking too, but have had a number of really great babysitters through the last few years. Our best babysitter we found through a friend with kids who loved her babysitter and passed on the contact info to us. But we also did some advertising.
The most successful one was: we lived (not anymore) in a college town, and there's an online job board for the college. We posted what we were looking for on that job board, and got amazing responses, and a few good babysitters.
Recently, since we moved to a new place and knew NOBODY, we used SitterCity, and while it's too expensive, and slightly annoying, we did find a couple of excellent babysitters through them. The whole experience is kind of weird, but in the end it was effective, and we were very happy with our sitter.