I second the milestones idea. I used to do this back when I was freelance web developing and it helps the client feel more comfortable knowing they're paying as they see progress. (Helps you a ton too cuz you get money as you go along).

This was one of the major features of my new startup matchist (http://matchist.com/talent). You create milestones for your client and they prefund each milestone before you start working on it. Once you're done, we release the payment to you.

Works great for both parties.