I know this is an age old question.... but to spend ten days of doing nothing and lazying around on the beach between 10 and 20 August

is the weather best in Phuket or in Koh Samui. Or is it terrible in both?

or Hua Hin?

There are lots of recent posts on this. JamesA gave the data on rainfall at all those places. I always use www.weatherbase.com

It will be very rainy in Phuket in August, while Samui will be high season, good weather.

August Samui would be the choice, you might still be lucky with accommodation.
If it's within the next 20 days ( which in you case it is ) Asiaoz and rates to Go list exactly what rooms are still available and they only show what 'is' available, it gives instant confirmation too so you can see just what 'is' available without having to wait around to see which place has rooms!!
Another bonus is you only have to pay something like 12% depost and a USD 3 booking fee, the balance you pay direct to the hotel when you get there, very good way of doing it at the last minute !! But is only good if you are booking within the next 20 days, so you can't put in until Aug 20th, put in say for 10th to 17th othersie it won't show anything

http://www.ratestogo.com/enter.asp?id=5604

I had a look for Samui to see just how 'full' it was, on this one:
http://b2b.ratestogo.com/enter.asp?id=5604

And that showed 17 places on Samui available from about USD 40 upards, Chaweng Cabana 65, ( superior garden bungalows USD 70 ) Amanda resort 53, Hua Hin they had from about USD 30, I don't know what price/star level you are after but a fair choice.
So you 'can' get somewhere if any of those on offer appeal to you.

excellent <james
<thanks
<<<<<<<<<<<<&l t;<<i think we'll try for Samui then. As we're coming from Europe and flights are extortionate now i think we'll look for a pacakage deal

i take it samui is much preferable to hua hin?

thanks

The 2 are very different indeed. Koh Samui is your tropical island, lively, nightlife, it's a 'tropical island', Hua hin is a mainland coastal area. If you want to laze on the beach then Samui, if you are more into say national parks, jungle, more sedate lifestayle, golf and such Hua Hin.
Transport is very different, for Samui you really need to fly although you can get train/ferry, bus/ferr combinations, only airline is bangkok Airways. Hua hin is only about 180 km south of BKK by road so you can bus if on a budget in about 3 hrs, or otherwise just cab or limo, about 2 hrs 30, which of course is a lot cheaper to get to, depends how you want to spend your days.