noseplugs!
salt water isn’t as hard on your sinuses as fresh water, but where ever you learn to roll, do use nose plugs! Goggles or a face mask are nice for learning a roll too, since they help you figure out where you are while upsidedown, but they’re not as critical as nose plugs. Some people say that you should practice without nose plugs to simulate real life, but sinus infections are no fun at all. And in real life, sea kayakers can also mimic white water paddlers, and keep nose plugs hanging on their pfd at all times, and use them when the conditions get rough and a capsize might happen.
This online description of teaching a sweep roll is long, but it’s an incredibly effective technique, in my experience: http://www.msu.edu/~connert/sweep.htm
By progressively lowering the learner into the water while she practices her sweep, the technique really helps imprint the proper movements without all the confusion that results from being upsidedown.
Have fun