To answer this most effectively, try these things first. Insert the butt plug, but do not fill it up all the way. Get it to a level where you can squeeze more air in when needed. Try the missionary position first as gravity will be your friend and help the penis stay in a tad easier. While you are on your back also try raising the knees up slightly. It may be more helpful to eventually raise your knees all the way to the chest as this aligns your colon in a better way. If slippage is still an issue consider putting pillows (or sex furniture) under your rear to tilt the vulva in a more favorable position. If this doesn't work, next try it in the doggy position but with a pillow under your hips to raise them up. It is all about angles and how your bodies fit together. I think if you try all of these variables you will come to a favorable solution.
