First off, is it a flat roof or what? IMO, a shake roof is cooler than an asphalt/fiberglass roof. But no matter how you end up going, one key is to have the roof as WHITE as possible. White doesn't absorbe the heat thus keeping the roof cool. Check out fiberglass shingles that are at least gray in color. If you have a flat roof, there are WHITE topcoat materials for this purpose. A lot of people like to have their flat roofs to act as a deck. Check around at ROOFING & SIDING Retailers. They're in the Yellow Pages under ROOFING MATERIAL.
You can check out these threads on Radiant Barriers. In sum, my $.02 says if you use a foil-based RB, be sure you have soffit & ridge/gable venting! The 1st thread asks about Radiant Paint but eventually, I/we 'get into' other RBs. Personally, I'd stay away from RB paint in the attic. Radiant Paints