In principle, the solutions above :
- setting desktop-provided power management -- or
xset -dpms ; xset s off for a one-time test
- disabling/uninstalling desktop-provided screensaver and checking/killing existing one to be sure (like
pgrep screensaver etc.)
...should be enough.
acpi=off to grub for this, it is off-topic here.
If all else fails, or you just want to do it once without changing all settings back and forth, you can resort to hacks.
For example, you can install xdotool (on Debian or Ubuntu
sudo apt-get install xdotool and you're done). This tool can simulate keypresses that will prevent screensaver and blanking just like normal activity does.
This command line simulates pressing and releasing left shift every 30 seconds:
while sleep 30 ; do xdotool keydown Shift_L keyup Shift_L ; done
I use that from time to time and it works great. It's also efficient at preventing laptop LCD screen backlight from dimming.
(You might want to change the key being simulated if pressing Left-Shift might disturb focused application.)
Don't let it too long for fear of damaging your screen (most screen technologies, when displaying a constant high-constrast image for a long time, eventually strain screen elements permanently. As a result, on flat-coloured areas a residual positive or negative image appears).