sleep positions

when using a machine, is it possible to sleep on your stomach? Sleeping in a different position would drive me nuts! Any advice would be a great help.

It's possible, but depends on your mask, your mattress and/or pillow arrangement, whether or not you tend to move around a lot at night, and your comfort level. I'm a tummy sleeper and use a FF mask... here's my story:-
What I did is get a 4" deep slab of hi-density foam rubber (make sure it's been aired for a couple of wks to get rid of any chem. fumes) which is on top of my reg. bed. At the top end, I cut out a 4" wide x 3" deep channel going towards the bottom of the bed x 10" long. My head, on it's right side, is mostly on my left hand, but my nasal mask rests in the channel. It works for me because I use an Ultramirage II and a Comfort Select, which vent not right out from the elbow angle like most do (and would be blocked by the mattress), but from a place at the bridge of the nose that's not blocked in the same way. It's not a perfect set-up, but it works for me, and without that channel, I can't get comfortable at all in the masks and they don't seal properly.