I would sugest that you make sure you have your back well supported by a parachute and/or cushion. i suspecrt this is more important than the amount of G or specific manouvers.
I used to have back problems and found that getting everything supported was key to being comfortable and not hurting my back. Specifically I found that I had to pull my parachute (backpack) down to the base of my spine, otherwise my back was in agony within 10 mins.