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This tip comes from linux-tipps from this article, lots more hints can be found on his blog.

I have a netbook that, when booted, may or may not be connected to an external monitor. The aim is to be able to correctly set up the system depending on what has been plugged in.

From linux-tipps, create the following file: /etc/X11/Xsession.d/98vgaonly, and make it executable (chmod +x).

Insert the following text into the file:

xrandr -q | grep 'VGA connected' && xrandr --output LVDS --off --output VGA --auto

This will check and see if an external monitor is connected. If it is, it cuts off the internal monitor and sets the external monitor to the best resolution possible. You can keep the internal monitor active, or set to a specific resolution. --output LVDS --mode <mode> will force a video mode, --output LVDS --auto will automatically set up the resolution.