There are several busses from Phnom Penh to Sihanoukville. KHR 10'000 one way or KHR 18'000 return. The trip takes about 4 hours with a short stop in the middle. They show an Asian movie and play loud music, so it might be a good idea to bring along ear plugs. A 500 ml bottle of water is included.
Most roads are in a very bad condition, though upgrading is under way.
There are several companies offering boat rides between Phnom Penh and Siem Reap. Prices are USD 12 to 20. The Cambodian government has assigned soldiers to guard these boats. The soldiers use the time on the boat to take a snooze, but you can make interesting contacts if you hand out cigarettes to them. Other reports state that somtimes the soldiers are shooting on the boats.