A solar-powered attic fan would be best.
- Get one that runs on a temperature sensor.
- They are readily available and easy to install yourself.
Adding a radiant barrier to the underside of your attic structure will help reduce heat gain in the space, but be sure to check your roof-shingle warranty and make sure it would not be voided by such a measure.
For more information:
Read "How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias." a Q&A answered by Danny Kelly.