I agree with princepawn - you will probably not get much use out of the contract, since a 'short-term one-off job' is not likely to get into the amount of money it would take to make it worth your while to try and enforce it if you have to.

I think getting some money up-front is your best bet. I have been told in the past that 50% up-front and 50% upon completion is a reasonable scheme - seems fair to both sides.

HTH