Originally Posted by 2MuchMark

For us, the best way we found to do this is payment on milestones. For example, if we tell a client that a job will take 5 weeks, we ask for 1/5 of the total payment each week, after we provide a demo and screenshots of the work.

This also helps in other ways. During each demo, you can talk to the client to make sure that the project is going the way they want it to go, etc. Very often, a customer will want changes to the project. Discussing those changes while the project is on going has always proven to be the best way in my opinion.