I sit all the way back, and strap myself in tight with the seat belt. I want to be as much "part of the machine" as possible. I don't want to be jostled about, rather be tight with the machine. I feel like I'm just too loose unless the belt is tight (sometimes start, forgetting to strap in).
My elbows ride hard on the arm rests and perhaps take a small bit of weight and allow me to adjust with the motion of the machine. Also, I have one foot firmly planted on the deck, with the other extended in a relaxed mode. If a large run (> 30 minutes) the feet get switched every few minutes so neither of them fall asleep.