Having been to Thailand 3 times now, I would definitely recommend checking out
agoda.com: Discount Hotel Reservations Worldwide - Smarter Hotel Booking! to book hotels in Asia in general. Good prices and works very well.
I always book hotels for my time in Bangkok and tend to wing it in the smaller cities.
Chiang Mai is good to see for a few days but gets boring pretty quick - seeing the elephants, the temple on the hill, etc. are all good things but not much night life compared to the "party" destinations.
I have been to Phuket twice and will not be going back - it is a crazy place with lots going on but it just way too crazy over the top for my taste. If you grab a moped and head out of town a little ways, it gets a lot quieter and a lot nicer IMO but the small towns seem to lack much of anything in terms of restaurants etc.
On the other hand, I will be going back to Koh Samui. The main party area / "strip" is Chaweng beach which I didn't really care for for the same reason as Phuket, however, some of the beaches outside of Chaweng such as the "Fisherman's Village" were beautiful and places I would like to return.
Hope that helps,
Mark