I dive up to 7 times per week and have waist length hair... so I have been thru everything. The best I have found so far is to do about 12-16 small braids around my head (not quite cornrows, but small, free braids). It is the most comfortable... however, you do end up looking like Medusa under water. If your hair is long enough you can actually "knot" all the braids in the back at the nape of your next and tie or clip them - GREAT!
Otherwise, if I am lazy, I just do one braid or french braid down the back. I also do a barette-braid combination, but it takes some time to figure out where the barette should sit so it doesn't interfere with the mask (right at the crown works best for me). Also, the barette rusts and you go thru a LOT of them. I use the Goody hair elastics that don't have the metal in them now for securing my hair, they are nice.
I have to agree with the comment about conditioner! I have found that if I put some "leave in" conditioner in my hair before diving, it is much easier to comb out and rinse afterward.
I have never had any success with the mask strap covers (they slip on my hair) or leaving my hair down (rat's-nest central). Low pony tails work, but I still have to braid the rest. Hope this helps someone.