You can install the dconf-tools package and then run dconf-editor.
In dconf-editor, open up the "org" branch, then "gnome" then "desktop" then "screensaver" and you'll see the name-value pairs that set the various screensaver properties.
Change "idle-activation-enabled" to unchecked.
You could also uncheck "lock-enabled" while you're there to prevent it demanding your password when it's been idle too long.
However this might not work! It didn't work for me when I was trying and I can only assume that there is a bug somewhere in xfce4, which is the windows manager I installed on 12.04 instead of Unity. If you also suffer problems, the work-around is to use xset:
xset -dpms s off s noblank s 0 0 s noexpose
but that only lasts as long as your session.
You can use
to see what the current settings are. It basically overrides the settings in /etc/X11/xorg.conf
You would have to put it into your .profile to re-apply it every session.