In a previous life I drove rally cars, and we ended up upside down all the time. I learned quickly that the secret is to reach down and grab the bottom of the seat with one arm, and undue the harness with the other. If you put your arm on the ceiling (now floor) and try to hold yourself up, you'll find it bent at an odd angle and it will instantly collapse, along with your body which will be a tangled heap. Holding the base of the seat puts your arm in tension (much stronger!) and allows you to control your, ahem... descent.
Do not underestimate how disorienting a roll over can be! Especially at night. (and I don't even want to think about water) As with most of flying, procedures, checklists, and training are your friend.
DEM