How-to Geek gives you the complete lowdown on how to do this, from recommended products to installation instructions and tips. The video above highlights the parts used their example, with a total cost of about $US65. A thermal fan controller automates the cooling so when your home theatre system reaches 30.5C the fans automatically turn on (and turn off at about 27C). The whole thing should be enough to cool two small cabinets or one large one.
Some great tips include putting the exhaust fan as high as possible in the cabinet because heat rises and placing the heat centre where you expect the most heat to be. You could either mount the fans in your entertainment centre or create a cheap stand for your fans using a wire hanger.
After completing this project, you can expect your entertainment equipment to not only last longer but also run more quietly.