Try to use Caffeine.

A status bar application able to temporarily prevent the activation of both the screensaver and the "sleep" powersaving mode.

Install:

sudo add-apt-repository ppa:caffeine-developers/ppa 
sudo apt-get update
sudo apt-get install caffeine

Try to use Caffeine.

A status bar application able to temporarily prevent the activation of both the screensaver and the "sleep" powersaving mode.

Install:

sudo add-apt-repository ppa:caffeine-developers/ppa 
sudo apt-get update
sudo apt-get install caffeine

EDIT: Currently, this method doesn't work on Freya.