I'm bloody hopeless at them, but from a technique DVD some tips are:

1) Practice the duck on land first until you can do it no problems. Learning on a wobbly board is a recipe for dunking. Back hand goes way down the boom, front hand crosses over and grabs the boom near the clew, then throws the sail back over your shoulder, so you can grab the boom in the normal sailing position on the new tack.

2) When you let go the front hand, the mast will go in the direction of the wind, so start the duck just before you head downwind.

3) You want to do a strap to strap gybe, don't try to step.

4) Peter Hart reckons that a significant proportion of people get their first planing gybe while ducking... hard to believe!!

Its all in your mind windy.

They are actually quite easy.
I pulled off the first one I ever tried and couldn't believe why I waited so long to try. And that was with a 7.0m sail.
Here are some tips.

1. Prepare the turn as if you are going in for a carve jibe.
2. As soon you feel the rig go light (i.e. you are running with the wind) lift your front hand off the boom and push the sail forwards and away from you with your back hand. Keeping the sail upright.
3. Now get your front hand as far back on the boom as you can get it (right near the clew) by crossing your arms.
4. This is critical. Be AGRESSIVE as you then drag the clew across your chest to the other side and extend your arm right out. Thats the duck bit.
5. Now the sail has turned in front of you (clew first) make sure that your free hand grabs the boom as far FORWARD on the new side of the boom as far you can get it (I.e. forward of the harness lines). . If you dont you will have to take a second stab at positioning your hands later in the turn and you will come off the plane.
6. Let go of the clew and grab the back of the boom with your (new) back hand.

(Practice this bit on the beach first to get your head round it)

Make sure you do all this while you are still running with no power in the sail. The flip happens fast and you have instant power which you have to control by staying on a run and not turning too aggessively.

Keep the turn shallow and remember to do all the things you need to do for a carve jibe. Keep your body weight forward. Just be very agressive with the rig flip because it is your muscles and not the wind that is going to flip the rig.

COMMON MISTAKE.
Turning too sharply and flipping the rig too late resulting in the sail being fully powered after you flip it.
CURE.
Get all the flipping done when he sail is light and you are running with the wind.

You will honestly wonder what all the fuss is about the first time you try.

Oh yeah and as Nebbian said, you are better off going strap to strap with your feet.

Gaz

Bout the only trick I can do too. I remember when learning them I crashed endlessly. I had taken to heart the advice that you can't flip the rig too early. Well you can. I certainly did, over and over. I had huge numbers of spectacular crashes. The good news is that I never hurt myself or broke anything. It was fun and it went a long way to inhibiting my fear of crashing.

Tips - pretty much as Gazza, Neb n Westy said but I might add that before you let go of the boom with the front hand move your back hand as far back on the boom as you can.

Nuther Tip - Robby does the sweetest duckies I've ever seen. The key to his elegance is a) he gets his old front hand right near the back of the boom and b) he really commits to the rearward throw of the sail. He rotates his whole upper torso backwards like he's about to throw a javelin. Brian Talma on the other hand does the dorkiest duckies I've ever seen because he is half-assed about both these points. He still gets them though.

If you do it right you have the booms in your hand on the new tack long before you are far enough through the turn to sheet in. So you have a nice moment there to switch your feet while doing nothing else.
Or you could step through a 360 and that's a monkey gybe.

Mr miller. If the rig is hitting the water you may be ducking too early. Get carving first. havn't done one in years, tried one today- Got it.

Hay there are a few main bits to a ducky,

go into it fast, over sheet as you start to bare away, like the beginning of a laydown jibe same stance knees bent and leaning toward the nose of the board.(biggest mistake with jibes is that most people are scared to lean foward and into there turn and lean back which sinks the tail and rounds you up into the wind on the exit of your turn, we have all been there)

it's important to keep your speed up and board planning so start out with a big ass turn so you can concentrate on the rig.

if you flip to early your mast will hit the water, unless you can muscle it back up, the best way is to flip when the rig feels light e.g your going faster than the wind, usually just before dead down wind.

flip the sail by letting go of your front hand and pushing the boom foward with your back hand, cross your front hand over as you go. grab back on the boom as far as possible, or i like to let the boom slide through my hand till i'm holding the rear boom end with my front hand. (if your game reach down and do a hand drag at this point, looks rad robbie naish style)

next is the grab, pull the boom across in front of your face and out to the outside of your jibe, you don't really duck under the sail. (just pull your head back so you don't smack your nose as the sail comes past) as your throwing the sail reach foward with the other hand and try to grab in front of your harness lines, this is the most important bit of the duckie, if you dont throw the clew of the sail out you won't reach it far enough foward it's important to do it in one motion, the sail will power up at this point so if you have your hand foward enough it should sheet out and pull you upright and the board usually falls off the plane at this point untill you have them dialed.

if you have flipped the sail at the right time you should be still standing and ready to grab the boom with your new back hand and switch your feet for the next run.

like all jibes look where you want to go e.g look into the center of your turn, and keep your weight foward.
once you pull a few you will realise there so easy and you will do them everywhere both ways.

Practice the sail flip on the beach with just your rig, digs a big hole but it helps heaps to get the steps sorted before the water.

Good Luck and keep sailing, the hottest guy on the water is usually the wettest.