Need a Contract

After reading the "Hostile takeover" discussion I started thinking. I have a fulltime job but occasionally I have been asked by other companies to help them with their IT problems after hours. This is not a problem with my current employer. The agreements for these after hour jobs have always been verbal but after reading the discussions maybe I need a contract. I have not had any problems so far. Now these jobs are rare and usually involve small amounts (under $500) so costs for legal advice is slim. My question is what should a contract contain and are there examples to follow somewhere on the web (even here)? Thanks.