Most people fly there by plane from Bangkok .
Have a look at the site of Bangkok Airways for more info on schedules and fares. You can also fly with Royal Air Cambodge, they charge 8.700 Bht and have 3 flights a week on wednesday, friday and sunday, departure in BKK at 14.30. From Vietnam Starting january 2000, Vietnam Airlines has started a direct flight from Saigon to Siem Reap 5 times weekly, namely on monday, wednesday, thursday, saturday and sunday. Check out their website, although it's possible they have not yet added this new info. It is nowadays possible to go overland from Bangkok to Siem Reap.