What I have found works the best for me is putting my backpack on my lap and then "threading" my seatbelt through the backpack's arm straps. Then I tighten my seatbelt. That way I have good access to my backpack, I don't have to worry about people stealing my stuff, and my backpack can't fall off or tip me over backwards.
I Carry most of my money in a side pouch. When I am out shopping I use the child safety straps on the grocery cart to strap in my purse. I was doing that even before my accident. It only takes a second for someone to grab otherwise