I always get 50% up front and don't do another until paid for the last. That is until I have a history with the client or have a very good reference, even then you have to be careful.

On another note, it is a good idea to do some work for free a few times a year. It is not only a good learning experience but good publicity and a charitable thing to do. You can even write it off if they will give you a receipt. I do one charitable shoot every year and get lots of new business out of it, well worth the many free hours spent on the project.