As a fairly new diver (< 100 dives) the biggest issue I had with back rolls was not the disorientation of hitting the water...I always knew that my partially inflated BC would bring me to the surface--and I wasn't going to drown because I had 3K PSI of air on my back. The problem that I had was actually "committing" to the roll. Hesitation will get you every time. Be sure that your fins are clear from other stuff on the boat (dive bags, weight belts, the diver sitting next to you accidentally having a fin on top of yours...). Make sure you have a couple of fingers on your mask, the heel of your hand on the reg and the other hand holding your weight belt (if you are using a belt on not integrated weights) or your gauges and anything else that can flop around. FALL backwards with your fins at a 90 degree angle to your body...DON'T hesitate...commit and go. Even if your mask leaks or you are disoriented, you KNOW with the partially inflated BC that you are going to come to the surface...you have all the air that you will ever need right there on your back and in your reg. Relax, let the disorientation pass and you are ready to roll!
Angie