There are often times when you want to completely override your Mac's Energy Saver settings. You could be giving a presentation and don't want your screen to dim or you could be doing work around the house and don't want to have to wake up your computer in order to see if you have any new email. For moments like this I rely on the free app Caffeine by Lighthead Software.

Caffeine is a great free Mac app that I use everyday. Clicking it prevents your Mac from automatically going to sleep, dimming the screen or starting screen savers.

Above is the app's preferences. Caffeine is simply a coffee cup icon that sits in your menu bar. Clicking it will activate the app and clicking it again will deactivate it. I keep my default duration to "Indefinitely" but you can set it at whatever duration you like. 

Enjoy!