1. On medium sized waves, say chest-head high, letting the front edge of the whitewater hit your lower back/butt area as you duck under can push you a little deeper before you have to level the board out and float to the surface. Once it gets much bigger than that getting blasted by the whitewater is more likely to rip the board from you hands.
2. On long period swells try to stay under longer before surfacing by keeping the board flat for an extra second once you are at your max depth. This lets more of the whitewater flow over you before surfacing. On short period swells most of the energy is contained in a narrower band of whitewater so get to the top and start paddling quick to build up some momentum before the next wave gets to you.