We once used the job board at a local college to locate a great girl. We called her references, and had her meet the kids a couple times in advance. She ended up sitting for us regularly for 3 years and was wonderful... and through her we sometimes used her roommate or one of her friends, so it was a good way to get a whole network of girls.
We also have in the past used employees of our daycare as evening sitters- some daycares don't allow this, however. We used the teenage daughter of a daycare employee as well. Also asking your friends for the names of their sitters is a good way to get someone who has already been "tested" by someone you know. In our current city we live in a neighborhood with over a thousand kids, so it was very easy to get hold of teenagers that are used by many of our neighbors, and through one of them we have recently hired her college-age friend to do our after school care.
The trick is to identify one girl you can trust, and then start branching out into her friend network.

To be fair, I shouldn't just say girls, I suppose, as we have also been very pleased with the teenage sons of one home in our neighborhood as well, but that is just less common (but very nice for my son as a change of pace).