I, also, request a deposit of 1/3, but I add another payment that is due with the client's approval of mock-ups. The final 1/3 is due upon delivery of the finals. My kill fee is usually equal to that second payment.

And a kill fee, for those who aren't familiar with the term, is a payment that the client must pay if the project is cancelled for some reason other than the artist's non-compliance.

I highly recommend that any artist who works freelance or on commission get a copy of the Graphic Artists' Guild's Handbook: Pricing and Ethical Guidelines. You can usually find a second-hand older edition (it's currently in it's 14th edition, I believe) for a few bucks. It has a lot of good advice for the business side of commercial arts.