Flat rate is a pretty good suggestion. Another idea is to use milestones. Pick a set of features, deliver by a certain date, they pay you, rinse and repeat.

Flat rate is a pretty good suggestion. Another idea is to use milestones. Pick a set of features, deliver by a certain date, they pay you, rinse and repeat. This reduces the risk on them, in that they won't be out for the whole amount. It reduces risk for you, in that you won't be limbo, if they take a while to get back to you on something.