A lot of heat can be trapped in the attic and make its way into the house. This is especially true if you're evaporator from your AC system is in your attic. When it becomes cooler during the night the attic can retain a lot of this heat. A good solution is to keep this space well ventilated. Several options exist for different prices and levels of efficiency.
a simple vent installed at the peak and soffit of the roof. Probably the most cost effective solution.
placed on the face of the roof and actively pulls air out of the attic when there is wind. A good addition to ridge vents but only effective when the wind is blowing.
very effective at pulling out hot air but they also use energy to run, offsetting the savings a little. Some models are attached to a thermostat in the attic and only run when necessary.
extremely energy efficient but the high initial cost makes it hard to reap the benefits. Wait until technology improves their design and the cost lowers.