This is something that I'm curious about also. When I look for a sponsor I first look at their sites to see if they have something that I think my traffic and surfers would like.

Once I find a program that works for us, then I read over what they have to offer and see if it is something that I think that my surfers will actually do. (Email programs, how many pop ups, ECT)

I will also email the company to see how long it takes to receive a response back from the company. Or see if they might post on boards, talk to webmasters, answer questions. I like having someone to talk to about issues that I have about stats, conversions, traffic ideas, ECT.