This is one of the reasons I always ask for a percentage up-front (usually along the lines of 30-50%). A client that is willing to do this will more than likely pay you when the project is finished.

Before I did this, I had tons of clients that just wouldn't pay me or lie and tell me the "check is in the mail"..when it really wasn't.

When I was young and naive 6+ years ago, it put me out of business and I had to get a job.