I've heard it discussed and seen it practiced on dry land. Lie on the back deck ofyour boat and roll over. You'll squash your belly, but will get the motion. I think all that the dry land practice is good for is to just get your oriented a bit.
Staying oriented underwater is the key to any roll. Try lying on the back deck of your boat and flipping over in water a few times. Just hang out for a few seconds. it will be like you are swimming, stretched out, face down in the water with a kayak that happens to be attached to you. (this is just a first, baby step in the process, that is all)
I learned the roll in an odd fashion. Originally, I learned a sweeping roll, like a sea kayaker, where I end up on my stern deck. Then someone taught me a more classic whitewater c to c or hipsnap roll. Then it moved into a real good combat roll in which my body always stayed forward. when I had all of that down, I just started rolling as I was flipping over, and combined the roll with my effort to get into that tucked forward ready position. Sometimes it turned into a back deck roll.
If you can, work towards learning how to roll off of any point of your boat. It's a long-term goal of course, but a good one. Eventually you won't be worried about back deck roll, sweep roll, c to c roll, or anything like that.
Here is another first-step excersize for the back deck roll. Assuming that you are rolling on your right side. Go to the side of the pool. Line up with your left side against the edge of the pool. Put your paddle down. Lean BOTH of your forearms on the edge of the pool. You'll be all leaned over and awkward at this point. Roll the stern of your boat over your head and make the boat land back in the water so that you are now facing the opposite direction, still holding onto the side of the pool. You will certainly use too much muscle at first. But eventually you'll be able to do this with very little effort. once it becomes easy. Try the same thing off of the bow of your friends boat. Or you can do it holding onto a friend/instructor's hands who is standing in the pool.