Contract! Contract! Contract!
Always get it in writing! Only a person who is begging for trouble will work without one. At the barest mininmum, submit a proposal outlining the proposed cost of creating the work and a rough time-frame of how long it will take to do it. Proposals are more loose than contracts, but just as binding when accepted. Actually, submitting a written proposal and after acceptance bind the deal with a contract nailing down all the approved specifics is better. However, you're going to run into clients that will prefer to roll with a proposal or contract or both! Really crazy but don't overlook that information submitted via an e-mail can be considered a proposal/contract! So be very careful and clear about what you put in your correspondence with potential clients.