In my case, I don't hold on to anything since locking the belt before each run. I'm sure the harness is thousands time better, but it's not in my budget right now(need tires first). At any rate, the locked belt provides a sneek peek at the advantages of having a harness. Then again, I'm not sure I want to have a harness in a top-heavy, sticky tired, stock soft suspensioned sentra that could flip over from too much grip(yeah it's not likely in auto-x, but I value my head).
Thought I'd help you out with a little item I use for Novice Autocross schools when I teach.
Go to a luggage store and get a luggage strap big enough to go around you twice. Then in conjuction with the seat belt technique wrap this around you and the seat and clip it. Then yank on the strap to tighten it down.