Nolo Press has a good (downloadable) book called Consultant and Independent Contractor Agreements that I used (too late!) to produce a good contract.

Don't do too much work before you get a contract signed, because without a contract it can be harder to get paid when things change. In my case, I put in about 80 hours of work (including contract preparation) and then found out that the reason they were so slow getting the signed contract back to me was that they'd hired someone in-house to do the job I was contracting for!

If you have to do a lot of work up front to estimate (which you may have to), try to get paid something up front.

And be careful that you specify the system in terms of behavior rather than in terms of any particular technology (because you don't want to commit yourself right now to a specific implementation).