As for the merchants themselves, I concentrate my primary merchants in 3 major networks - CJ, Linkshare, and BeFree. I then pick up other merchants as "filler" in secondary networks. When reviewing a merchant, I look first for a business plan that seems reasonable. I'm also very aware of how much time and effort they've put into their site and the merchant plan. I do not chase the highest CPC, but will -and have- chosen the lowest paying merchant because they have a known brand or good reputation among affiliates. Finally, and most importantly, I rely on "buzz." If there's a WHIFF of a problem from a reputable source (opinions from either Drastic and Czar are highly regarded, but there are a few others), I don't bother with the program.
If you are terminated by a GOOD program, write them a "professional courtesy" letter asking that they review their decision. I have lost two and been reinstated by both.