Hand rolling your whitewater kayak is a really useful thing to be able to do, whether it's because you broke a paddle, got it knocked out of your hands, or threw it to the side after securing your boof on a 90-foot waterfall.
Ha! Oh wait, I forgot - not a joke for some people. Anyway, you may have noticed that some kayakers make it look incredibly easy, and I'm sure it can be, but learning it can also be a real challenge. Here's a good instructional hand roll video from Eric Jackson of Jackson Kayaks.
I actually made a hand roll on my very first attempt a couple of years ago, but after that it was hit or miss (mostly miss) and now my form has morphed enough that I'm not even coming close. That being said, I haven't watched and followed the tips in this video closely.
Ken Whiting has a good article on Paddling.net that shows and explains more of the technique. Here's his video showing how to hand roll, which I think is also very good. Yes, he is in a sea kayak, but try to ignore that, the concept is basically the same.
One thing that will help you with that, aside from the actual muscle memory derived from doing that drill at the side of the pool, is doing side stretches to improve flexibility. Every couple of degrees by which you increase your flexibility makes a difference in how long you can keep your head down as you roll up, and little things like that add up to critical mass surprisingly quickly.