How do I completely turn off screensaver and power-management?

For 11.10

Disable the GNOME Settings Deamon's power plugin from the command line:

gsettings set org.gnome.settings-daemon.plugins.power active false

Alternatively, using dconf-editor from the dconf-tools package:

For the screensaver:

gsettings set org.gnome.desktop.screensaver idle-activation-enabled false

Or again via dconf-editor:

You might find that the above solution is necessary, but not sufficient. The screen might still be blanking after 10 minutes. This is a default setting compiled into Xorg.

To disabled it for the current session, run:

xset s off

You can put this in your .xsession file to disable it upon every login.