To use a specific resolution, or turn on or off certain displays by default when you log in, you can create an
.xprofile file in your home directory which runs
xrandr commands. For example, this
.xprofile will turn on an external VGA monitor and set it to 1280x720 resolution and then turn off the internal LCD monitor on a laptop.
xrandr --output VGA1 --mode 1280x720
xrandr --output LVDS1 --off