Hi spwalker75,
Paul from Angie's List here. Gadgetgirl makes some good suggestions. Typically, we recommend prepayment between 10-15 percent of the total value of the project, but it can vary. Whenever possible, avoid paying for a project with cash. If you can, use a credit card instead so you recourse in case something goes wrong. Before you sign off and make a final payment, check to be sure the work is complete and to your satisfaction.
Good luck with the project!