http://forums.soundandvisionmag.com/audiovideo/board/message?board.id=57&message.id=7630
that should get you started ,,make sure they are at least an inch and half off[away] the side walls ,,make sure you wrap acoustic treatment with poly batting then cloth ,,and using the mirror method works so make sure they are about 2 feet by four ,,I prefer 2 feet wide ,,and at lesast 1 and half thick absorbtion ,,for reflection down to 500 hz ..mine are a foot and half off the ground and work great .
and yes one off the center ceiling helps too even in a non ht system..