Paint your roof white
A study by the Lawrence Berkeley National Laboratory's Heat Island Group found that in sunny climates, buildings with white roofs required up to 40% less energy for cooling than those with black roofs. At current utility rates, that means you could save more than $100 per year in cooling costs.
An inexpensive white, elastomeric coating will do the job and can be found at most hardware stores in states in the southern half of the U.S. Elastomeric coating is a blend of polymers that is durable, flexible and waterproof, and offers the fringe benefit of helping to increase your roof's life span and water resistance. All you'll need to apply it is heavy-duty paint rollers on an inexpensive old paint roller frame, with an extension pole attached to save wear and tear on your back.