Matt Laur , Jun 03, 2010; 08:59 p.m.
You don't mention what country you're in, so it's a bit of a guess about what laws may apply ... but: typicially it's the advertiser (not the photographer) that can get in trouble for using someone's likeness like that without permission. Advertisers will frequently turn to the photographer as a potential source for that signed release document, but you're not going to be in hot water personally because someone else decided to mis-use a celebrity's likeness in an ad.
Yes, on your last question: you need to include specific license language in the paperwork you do with your clients. The wording of such licensing can get very complex (including and excluding specific uses, time periods, morals clauses and all sorts of other things). If you join a professional society, you may be able to get some resources along those lines, or ... hire a lawyer to help you draft your routine language.
I'm not a lawyer, I'm a caveman. This isn't legal advise, it's cave paintings.