Use a technique learned in a ski class. We spent first 15 minutes learning how to get up, still works great. Roll over, get on hands and knees.
Edit. Oops, did not finish this. Get on hands and knees. Move hands toward knees, [edit again: get feet under you, one at a time], get butt up in the air, and then ... stand up! I tried this just now on bathroom floor with towels under me (clothed, in case it didn't work and I might have to call for reinforcements, lol) and it worked a treat, even with my sore knee. Recommend practicing.