Hi, for mplayer you can use this configuration:
heartbeat-cmd="xscreensaver-command -deactivate &"
Place it in either ~/.mplayer/config or /etc/mplayer/config
It should work with gnome-screensaver.
For VLC, open [Tools > Preferences] at the bottom left of the menu click "all", there should be an option "Disable Screensaver" under the video category.
If you applied those settings and the screen still "blanks", you might want to run this in a startup script:
This explains the above command nicely: http://www.shallowsky.com/linux/x-screen-blanking.html