Seriously, I've trained myself to ride Green Lantern at SFGAdv as follows for minimum pain and maximum comfort:
Front "seat," pulling down on harness to keep it as close to your body as possible while standing on tippy-toes. I don't like standups, but I can enjoy GL using these maneuvers. Can't speak for RR (never been) but I imagine it's similar.
When you board the train, jump up onto the seat and lower it until you are standing on the floor. Then, lower the harness and buckle it just like you would on any other B&M. At SFMM, the seats are not locked until just before departure, and it is announced. After your restraint is checked, place your feet flat on the floor, bend your knees slightly, and wait for the seats to lock. Once they do, sit on the seat slightly to ensure it locks into the lower setting (each setting is about an inch apart) and you aren't suspended slightly during the ride. Finally, stand back up, hold on, and enjoy the ride. Just be sure not to lock your knees, and keep them slightly bent to absorb the forces.
For the smoothest ride, stand in the first three rows. For maximum intensity, pick the back two. I personally prefer Riddler's Revenge in the front, so if the wait is not excessive you might want to try riding up there. Make sure to count people instead of looking at the length of the lines, however, as the bizarre slant in Riddler's Revenge's station can make a line that appears shorter actually be three or four trains longer.