Hi,I'm going to thailand for three weeks on sunday.We are staying in bangkok for the first few nights and planning to do some travelling up to chang mai and then down to the islands koh samui,krabi(depending on weather!).
Can anyone tell me if it's possible to get flights from chang mai direct to any of the island without having to go back to Bangkok and then fly down to southern thailand.
Thanks!

Unless things have changed dramatically since I was there, you will not only have to go through Bangkok from Chiang Mai to Koh Samui but you will have to change planes and airlines as well. The only airline that flies to Koh Samui is Bangkok Airways. Thai flies from Chiang Mai to Bangkok.

What Craig has said is correct but I will justupdate and hope it won't sound complex:-
Thai Airways flies most domestic routes.
Flights are frequent ( e.g Chiang Mai 12 x daily, Phuket 14 x daily - mostly all flights are wide-body aircraft)
Bangkok Airways has a couple of flights from BKK to Chiang Mai ( direct and via Sukhothai ). Bangkok Airways is however the 'only' airlines that flies to Koh Samui.
If you are heading south from Chiang Mai you will have to go back through bangkok except for a once daily Thai Airways flight from Chiang Mai to Phuket.

By road from Phuket Airport to the beaches around Krabi is about 2-2.5 hours.

Bangkok-Chiang Mai / Phuket / Krabi are all about B 5,500 +/- return. Bangkok to Koh Samui though including airport tax is about B 7,700 return.

Bangkok Airways also flies 3 x daily Bangkok to Phuket and Phuket Air has 1 flight per day.

You can buy a multi-sector deal from Thai but you MUST buy this before leaving, it gives you say 3 sectors at $59 each. This is a good deal if you do BKK-Chiang Mai-Phuket-BKK. You can call Thai Airways and ask them about it.
This deal is good on those sectors but not such a good deal on shorter flights.

You best bet is just buy a BKK-Chiang Mai one way as returns are simply 2 x a one way ( excursion fares etc are rare ). Once you are in Chiang Mai do the wise thing you suggest and check the weather in the daily Bangkok Post newspaper. Then decide if to take the non-stop to Phuket or head down Koh Samui way.

Bangkok Airways 'was' offering some $50 per sector deals but you would need to check their website and I'm not sure they are much good if you want to retain flexibility.

Trains also operate to Chiang Mai frequently, make a change! You can train/bus/boat from Bangkok to Koh Samui.
Overnight VIP buses to Chiang Mai/Krabi/Phuket cost roughtly $17 one way. They are 'fast' and not that comfortable. if you do choose to use the bus services avoid agent operated so called VIP buses, stick to the Thai operated VIP buses which are far better!

Ditto the above. Thai Air also has flights directly into Krabi rather than taking the drive from Phuket.- although the road is not bad and the drive from Krabi to Phuket was well under 2 hours.