Use your hands. Grab anything bolted to the plane to keep you stabilized.
If turbulence is severe, you may lose your grip. Once on a United flight I was walking down the aisle when we were hit with pretty rough turbulence. All nearby seats were filled and it was too rough to walk to my seat. A flight attendant told me to lay in the aisle and loop my arm through the lowest part of the nearest seat frame (you know, that metal bar that keeps your personal items from sliding forward) and hold my forearms with my hands. Thus, I was hooked in. I was bounced around a bit, a little bruising on the inside of my elbow where it went through the frame, but at least I didn't hit my head on anything or hit anyone else.
So, if you are out of your seat and surprised by severe turbulence, drop down and hook in.
That only happened once in a few decades of flying. As others say, it is rare.