Koh Kong is linked to Phnom Penh and Sihanoukville via Sre Ambel by National Route 4. The road is now completed including all 5 bridges (May 2008). It's a good scenic drive through some of Cambodia's least developed and unspoiled regions - Cardamom mountains. Minibuses and tourist air-con buses leave Koh Kong mainly in the mornings (but some do even in the afternoon, probably depending on number of passengers available). On the way back, they usually leave in the morning and arrive to Koh Kong in the afternoon. The proper price is 25000 riel (6USD).About USD8 or 10 is a price often asked in Sihanoukville travel agencies for the ticket back to Koh Kong).
There is no longer a boat from Koh Kong to Sihanoukville. It stopped in 2009 although some travel agents in Thailand still sell travel tickets for the (non existent) boat.
Currently (Jan 2010), there is a daily bus leaving to Koh Kong from Kep for 20USD. It leaves in the morning at 8:30.
From Thailand
The Thailand border crossing named Cham Yeam is about 10 km (by the road) northwest of the town. Motos (normal price is USD 2 to 2.5), shared taxis and taxis (USD 9 to 10 for the entire car) go between the border post and the town/port, and the drivers of these will attack you while you're waiting for your visa. You may and will be asked for much more (400 baht is quite common) - just bargain a lot, and do not let them to rip off you. Additionally, you'll have to pay toll fee for the bridge, which can be paid in either baht, riels or USD. The ongoing rate will be stated on the toll booths just before the bridge - better pay this on site and do not include in your taxi fare, to avoid a chance of being over- or double-charged. When in town, the driver will offer you to exchange some money to riels - just refuse, the rate would be very poor, USD are accepted universally in Cambodia, and, in Koh Kong, Thai baht are accepted too. If you're continuing on to Cambodia, he'll also try to convince you to buy a bus ticket for an inflated price, USD15-30 or more - just do not agree, or you'll be charged this price on arrival to the agency, and bargain when he drops you there. In the agency, you may be told that the normal price now is USD25 or 30, and, as such, USD15 is a big bargain - this is not true. The proper price to Phnom Penh is 25000 riel (6USD) if you get a ticket for 8USD you are doing well. There is a good friendly bar, Fat Sam's, in the main street where you can get good honest tourist information as well as the best Western food in Koh Kong.