One of the best FREE and easy-to-use apps I've found for this is called Amphetamine. The Amphetamine App (a great substitute for the the popular Caffiene app) will help you do away with the annoying computer snooze...
The functionality of this app is the best part. There is no need to take the time to navigate your way to the computer's System Preferences and settings. You can click on the icon from your home-screen and it stays aways instantly.
You can get the app for your Mac here: itunes.apple.com/us/app/amphetamine/id937984704?mt=12