Do you have an attic? If so, consider a whole house fan. It installs in the ceiling. Turn it on in the evening, when it gets cool outside. The fan sucks cold air in through the windows and into the attic. This forces all the hot air that is stuck in the attic out the attic vents.
Here are a couple of sites with some info. Note that you can get a fan cheaper at your local home depot, and install it yourself.
http://www.lowes.com/lowes/lkn?action=howTo&p=Improve/wholehousefan.html
http://www.quietcoolfan.com/index.html
http://www.wholehousefan.com/