I usually look at BP just to see what's new, but am pretty cautious about booking anything from there unless they're a regular provider. After that I take a quick look at the pink board, then a couple of the agencies, EH and Mirage.
Once I've seen who's advertizing/available, I look here for reviews, however I find that unless there's someone new and with a lot of positive chatter on the boards, I usually stay with my 3-4 regular providers.
My preference is always to find providers who are local and have an actual plan, like providing for a family or working her way through school. I'm not a big fan of giving money to someone who's only here in town for a few days.
Naturally, I stay far, far away from anything that even hints of being human trafficking or someone who's working against their will.