Dude, if you need to know how to duck-dive, perhaps you should stay away from those "nine foot sets" for a while...
There's a lot of info on the Web to help you on duck-diving, but here's some thoughts... when it comes to the bigger stuff, duck-dive deep starting with your (board's) nose, hands on the front corners, and then press in with your knee to encourage a scooping motion under the wave. Use the other leg to balance yourself, kinda like the way a new-born kitten uses it's tail.
The trick is to stay under till most of the turbulence has passed over you, else you will get dragged when you come up - you need to stay calm to do that. Sometimes, if you have the strength left, it helps to kick a bit in order to prolong your stay under the wave...
Some waves it's better to duck-dive a bit late, cos they bounce over you, but that's sooo difficult to work out before-hand. Also, if you're gonna bail, hold onto your board's nose if you don't wanna snap your leash - and remember - if it gets hairy there's no shame in bailing, if it's going to save you from exhaustion, which can kill.
Peace!
