Favorite Technique for finding voice

One of the things I continuously need to work on is making NPCs sound distinct. What are your favorite techniques for doing this?

Sarah Jane Adventures

Sarah Jane Adventures is the best Doctor who on Television. Cool and no melodrama (well not that much). Sarah Jane is amazing, and the kids aren't so bad either.

I'm in love, which is a good thing because both Torchwood and most of season 3 left bad tastes in my mouth.