Either put something like this in your xorg.conf
Code:
Section "ServerFlags"
Option "BlankTime" "20"
Option "StandbyTime" "0"
Option "SuspendTime" "0"
Option "OffTime" "30"
EndSection
... set time values as required, 0 to disable.
Or use the "xset s off" to disable it from the command line (or add that to your ~.xsession file.
"man xset" for details/options.