i don't think it has been said yet, but if you use a projector screen you should paint the side walls and maybe the ceiling a medium to dark gray to absorb reflections so they don't wash out the image on your screen.
this should be done near the screen going towards the audience.
no need to paint the whole room.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery