Getting held down after a wipe out can be a nasty thing, especially if you run out of breath halfway through. Here’s a tip from Bill, via the Zone, on how to increase your breath-holding time. I’m gonna do it!
I’ve just come across a breathing technique that more than doubled my breath-holding in one day. I know that sounds ridiculous, but I’m headed for three minutes after struggling with one just before I found this. I did 2:43 yesterday. 
You fill the bottom of your lungs first, by pressing out forcibly with your stomach as you start to take a breath then expand your lungs. It’s a little hard to describe the full cycle, but you can practice it anywhere and you’ll get the feeling–it’s kind of a rolling breath and you push downwards with your diaphragm.
I’m writing an article on it for Ke Nalu, but the rest is just refinements. If you take four long breaths like that you’re good to go. What it does is use your full lung capacity instead of just the top. Shallow breathing just moves air up and down your trachea, and leaves a lot of stagnant air in your lungs.
How this applies to surfing is that I’m practicing taking these kind of breaths all the time when I surf. And when a big wave is coming that I aim to take, or I’m caught inside, I breathe this way. I can tell you first hand that it works well. the last two days have been rough for hold downs. As I said yesterday, two evenings ago I got caught inside the channel at Kanaha and held down forever. I probably would have made it anyway, but this technique transformed it from a grim battle to an interesting if unpleasant experience.