A minibus runs from Sihanoukville to the Thai border at 7.30am daily. This service is run by Mealy Chenda guesthouse in Sihanoukville.
Boat departure from Sihanoukville to Koh Kong are at 12pm each day. On arrival in Koh Kong either stay the night if you want or just get a pick up truck or motorcycle to Thai border or before 5pm if you arrive with enough time to spare. More hazardous speed boats to the border operate from the ferry immagration stop before Koh Kong. These are not recommended in wavy conditions.
After crossing the Thailand border a regular B100 minibus to Trat leaves regurlaly and takes just over and hour.
If you are cooming from Hat Lek on the Cambodian border as I was, you will be deposited at the new bus station on the outskirts of town regardless of your means of transport. In fairness, it is not too far out of town and you could walk it, but after a long day travelling from Sihanoukville or Phnom Penh, you will probably want to avail yourself of the services of one of the shared taxis there. They are the blue, jeep type affairs and you can negotiate a price as low as 20B into town although I think that was part of an operation to deposit you off at Pop's guesthouse (see seperate tip). Once there, however, you are free to wander off and make your own arrangements.
Please arrange with other people to share a songthaew from the pier (mainland) to Trat. Otherwise taxidrivers won't even try to get you to the city. Unless you want to pay a very high price ofcourse. Yep, I arrived at the pier way to late, and nobody else was there to share a cab with. So I payed the 220 bath all by myself to get to Trat. Ok, it's quite a long distance and takes a while to get there. But if sharing with others, you pay about 50 bath each.