Ther are loads of small tour operators in BKK that will book your mini bus/ferry transfers to Samet and Chang the day before you want to leave.I always use a great one that is just near the Biokie Sky hotel in Prathanam(SP?) Works for me as I always stay in this area of BKK.
Kaw san road there will be plenty to choose from but the prices are about the same wherever you leave from in BKK.

If you book a return ferry trip from Koh Chang from a tour operator in BKK then make sure the guy in the mini bus gives you your return ticket before you even get in the mini van.

There is a bit of a scam with the tour guide (The person who shows the mini bus driver where the hotels and pickups are) And the guys at the pier,when you get to the pier you are told that the ticket you have is only a one way ferry ride and you must pay the return price now or you cannot get on the ferry.The guys at the pier can get quite agressive and demanding and ask for a lot more than the ferry actually costs. Not so at Ban Pae pier for Samet,I have never had a problem.

good luck and have a great time.

Den