I didn't want to just say "read the manual", so I tested this with my two monitors at work (VGA and DVI, the LVDS -- laptop screen -- is closed).
man xrandrto get more background information. I'm only going to touch on the relevant basics, here.
xrandrby itself will list your available screens, and their modes (you did this already, in a previous post). Modes with a "+" beside them denote the default mode. Modes with a "*" beside them denote the currently active mode.
Based on your previous post, your laptop screen (LVDS1) is in 1280x800 (which is it's preferred mode). The projector (VGA1) is in 800x600, and has no default.
If you want your laptop on the left, and the projector on the right, try this:
xrandr --output LVDS1 --auto --output VGA1 --mode 800x600 --right-of LVDS1If you want the laptop on the right, change "right-of" to "left-of". Other valid values are "above", "below" and "same-as" (clone).
Try these, and see if it forces the screen layouts to what you want.
Then, we can take it from there...