This comes from Nate Burgoyne, via the Zone, who offers up some great advice about what to do during a wipeout.  Read it and do it.  I know I will … I hope.  (P.S. Please see end of post for links to Nate!)

In  heavy wipeout, in all seriousness you gotta stay calm. Honestly, the staying calm part begins far before you even wipe out. If you are paddling out to the lineup and while you’re in the channel you see the bombs rolling through and you start paddling like mad for no reason and you’re heart is racing, you probably shouldn’t be out there. You’re already panicking. However, if you’re paddling out in the channel looking at the bombs pitching over and you feel calm, collected, and it all puts a smile on your face, you should be alright.
When I do go down after a closeout, unless I get totally rag-dolled, I personally straighten my body out and let the wave drag me feet first as long as the waves wants to. I feel that if I resist, I spend oxygen that I need.
Also, ever since I’ve decided to just go with the wave, leash breakage hasn’t been an issue. Keep a good grip on the paddle and you can control your direction somewhat underwater. You gotta go with the energy of the wave. If you try to resist or to fight against it, wasted energy.
Also, since paddles float you also always know which way is up. Once things calm down, I get to the surface for another breath to try and avoid the possibility of a 2 wave hold-down. Also, when you come up, don’t race to try and get your board if you don’t have to. It’s wasted energy because the next wave is going to take it from you anyway if it’s a sizeable day.
You also need to be able to hold your breath and stay calm with water in your mouth and nose.  For me, free diving in the summer helps a lot as well. Sometimes when you get slammed, if forces water up your nose and in your mouth. Free diving for me, taught me to be comfortable with that. The natural reaction is to blow it out but if you do, you just lost your air.

Here’s a link to the post: Best Breathing (or lack thereof) Practice for Hold Downs?

Here are links to Nate himself:

Editor of Stand Up Paddle Surfing Magazine www.supsurfmag.com
Internet Marketer, Educator, Web Designer www.nateburgoyne.com