>>edit: i guess i wasnt clear enough. Im not selling a computer, a service. I should just get his address, that way if he doesnt pay, I'll kill him ::evil laugh::<<
it all depends on whom is "buying" your service.
most companies, including DoD (the company i work for) WILL NOT pay for services until they are completed and reviewed for completeness and satisfaction - then and only then is payment made.
if an individual party is "buying" your service, do like rod or ober suggested, or you can even ask for half up front, and the reaminder upon completion.