Just want to raise a subject that the newer riders may be interested in and the older heads may have some comments on.
I've been caught out with larger gear and been hit not by gusts so much as a wind change and a much stronger wind than what the kit was set up for.
Something I've tried and seems to work is to pull your back foot out of the strap, unhook your harness and get your weight over the board. In this position you are not using the sail for balance and can then rotate it so that it is not catching the wind. From here you can "feather" the sail and it won't pull you over but still have enough power to get you home, even planing if you want to.
One time I was hit with a really heavy wind and knocked clean off the board and no matter what I did could not re-start. The board would round up, or I just couldn't get everthing in the proper position to water start. In this case I just got the sail in a riding position and it pulled me back to shore much like a sub-planing body drag. Not pretty but at least I managed to get in to sail another day.
For newer riders I think there may be some reservation to get sufficient sail up but if they knew there was a way to safely get back in they may be encouraged to try a little more sail.
Just my two cents.
If extremly overpowered I would be wary of pulling my back foot out of the strap, it seems like a recipe for a catapult to me.
If you can manage it, I would have thought that unhooking would be safer. At least that way you can let go if you have to.
The experienced crew will possibly have a better solution than me. Only been sailing a few seasons, and always sail well powered up bordering on overpower
Solution that works for me is to out-haul as much as possible, keep the down-haul at max, and endeavour to remember to push the back hand away hard in the gust, reducing the drive over the sail surface.
Can be a mongrel trying to watestart when its howling, I agree
O, and I always wear a buoyancy vest, in respect for the water and family concerns, no matter how placid the conditions are
I was majorly overpowered today. To the point I had to seek shelter on the wrong side of the harbour. I waited there for about 50 minutes for the peak gusts to die down then came back in the straps but unhooked. I find I can get lower and further out when unhooked. As said above, being unhooked allows you to take a bigger chance with sheeting-in when overpowered.
Thanks to the guys at the local Concord & Ryde sailing club who were nice enough to come over to see if I needed a lift back across the harbour.
I was out yesterday on my KA Koyote 6.6 in wind that was gusting from 25-40kts (according to the graphs on seabreeze).
I was having some major problems waterstarting too, but once I got on my board I was putting my front foot up against the mast foot and my back foot just in front of the back footstrap. I was hanginng like a monkey, as Guy Cribb says, because it helped keep my weight over the board (to survive the lulls, when the wind literally dropped to nothing) but still be low enough to deal with the gusts when they came.
Also, having my foot up against the mast foot helped deal with the gusts, because I could push the power through my front leg forward into the board, helping it get planing so I could get strapped in.
The Formula guys might chip in here, but if home is pretty much downwind, take your back foot out of the strap, unhook and do a quick half gybe to almost directly down wind. Try and keep your speed at about 20 knots or a bit less. If you get going a bit too fast go deeper, if you start slowing down head back up for more speed. When going downwind the gust/lull ratio of the apparent wind is exaggerated so it's a pretty wobbly sort of thing to do - but the apparent wind is down from the true wind by close to your 20 knots of board speed, you're not going that fast so crashes are pretty mild. The back foot out of the strap allows you to easily bear away as a gust hits.
The thing to avoid is losing speed in the lulls, because if you're going slow when the next gust comes it will hit hard. So be always ready to head back on the wind the instant you sense boardspeed dropping
Practice it next time you're out in fully powered conditions, it's good fun. Maybe you all know this, but I certainly spent many years baffing unaware of the possibility of going deep downwind in overpowered conditions.
the only way you will get back home is to point into the wind as much as you can. so you use less sail area and tack like a monky back and forth to your beach or else next stop is tassie, if ya live in melbourne, dudes. another thing you can do to i was reading about a storm chaser in the states, who chases storms to windsurf in with his 3 m sail and a fanatic 60L plastic board for weight. he said if all else fails, you can poke a hole in ya sail to let out abit of air. so ya 7m becomes a 5m and you can at least windsurf home. 
There's overpowered and then there's overwhelmingly overpowered to the point of barely being able to stop the gear blowing away.
In the latter case, hanging like a monkey may not be the best plan, as you'll still be sheeted in and at the mercy of the gusts; I had this happen recently and found if I unhooked, came out of the straps, stood on the board like a surfboard and sheeted out all the way (with a cambered sail) , I was able to plane almost gracefully back to the beach in time to grab a smaller sail...
Reading the comments on the thread, I may of mislead some of you.
Wet Willy is spot on with his comments. When I wrote overpowered I meant OVERPOWERED to the point where it is survival. You cannot stay in the harness because it just sheets in the sail and you are just going too fast. The board gets too much air under it and just takes off out of control. In these conditions I found that I had to get my weight onto the board and off the sail. I've found the conditions in Tassie are not the same as in QLD where I learnt and I've had to discover how to sail in these very changeable and challanging conditions. Does anyone have any other ideas when all you want to do is get in in one piece?
Ian is on the money here, when super overpowered either - bear way off the wind, or sail very steep/high into the wind, on these 2 angles you will find the easiest, its the square reaches that will frighten the bejesus out of you. It does take guts to bear away. I tried this a few months ago with a 6 m sail in 30 knts in open bay conditions and it does work. Peter Hart did a series of articles in this just recently in downwind racing / trips in UK Windsurf. But there will come a point where it will be unsailable. Hopes this helps Icey 
I had exactly that way, way overpowered experience at Sandy Point on Saturday when it must have been gusting up to 50knots and I had a 4.7 rigged. Earlier in the session it was maybe 30-35 knots and we blasted along OK, albeit with the wind getting under our boards on speed runs leading to lofted board then buried leeward rail....
But late in the session the wind got so strong that all I could do was let out my back hand to spill wind which meant that I ended up falling backwards into the water and then had a hellish time of waterstarting as mentioned earlier in thread. It didn't help that my arms were knackered from nearly 3 hrs of sailing and the extra exertion of many failed waterstart attempts.
I ended up blown to the far bank nearing the onset of darkness and hobbled back to the shore with exactly the technique described by Iceman - no harness, no straps, upright stance. Not pretty or stylish but it got me back to shore.
Like Mineral, I always wear a bouyancy vest, and in manageable winds, as a bonus, it aids waterstarting . It is also good padding for crashes on speed runs.
i'd like to say in really overpowered conditions maybe go back to the basics like flutter the sail in neutral with mast in between your feet while trying to get balance and when a manageable wind comes sheet in and hurry like hell back to the beach
