I saw a link to these Useful Legal Documents for Designers on Twitter the other night from AIGA Raleigh. My students are always asking about contracts and business practices. This site has compiled some nice sample agreements, available to be downloaded for free.
These can be great a starting point and help you figure out what needs to be covered in a contract. But I recommend that you work with a lawyer to customize it to fit your needs. Laws vary from state to state. And your unique situation may require a different agreement.