Draw up a contract that details every step of the project: payment schedule; proof of liability insurance and worker's compensation payments; a start date and projected completion date; specific materials and products to be used; and a requirement that the contractor obtain lien releases (which protect you if he doesn't pay his bills) from all subcontractors and suppliers. Insisting on a clear contract isn't about mistrust, Tom assures us. It's about insuring a successful renovation.
Finally, remember that as soon as a change is made or a problem uncovered, the price just increased and the project just got longer. The four most expensive words in the English language? "While you're at it...."