By rail/road:

From Phnom Penh

Taxi:  Departure   flexible; 4-4 ½   hrs journey time $45-$50 Up to 4 passengers (price variable & negotiable)

Buses: Mekong Express and Hong Rong Kheang at the best on this route. Both are air conditioned with on board clean WCs and are well driven: cost $6 Hong Rong Kheang $9 Mekong Exp (with light meal and drink) Buses depart from near Phnom Penh Port 7:30 a t 12pm They   take 5 ½ hours to the Siem Reap Bus station 3 kms east of town.

Overland from Thailand: The overland routes into Cambodia are generally improving; however even the main routes to Siem Reap a in a very poor state of repair in places. Take the air con public bus from Bangkok (Morchit) Northern terminal.

If you catch a bus at   7am or 7:30 you can get to Aranyaprathet at 12 pm.   A short Tuk Tuk ride (5Km) takes you too the Cambodia border.  

Train from Bangkok is a good and enjoyable option. Trains leave at 5:55 and arrive at Aryanapathet around 12pm. There are many willing tuk tuk drivers to take you the 6kms to the border.

Don't be persuaded to buy onward tickets on the Thai side as you are likely to have long delays waiting for a bus and you will be likely to suffer the same fate as those buying a ticket from Bangkok,

Poipet Border Crossing; the crossing situation changed in July 2005 . The price of a visa has risen to 1000THB ($25). On exit from Cambodian immigration, tourists are sometimes directed to a 'free' Tourist bus which is now taking tourists to an a new Bus station   near the Market in Poipet; where minivans and buses transport tourists to Siem Reap.  It   is not compulsory to use this .

The fixed taxi fare has risen to $45 for tourist cars to Siem Reap. This has been enforced by the authorities by putting up the fee that drivers have to pay per journey .

  The  cost on the bus service to Siem Reap is   $11 per person. The bus is ending its journey at the new Bus Station east of town. There can be delays waiting for the bus to fill up; but the bus drives direct to Siem Reap   Bus Station; with a short stop and is taking 4 to 5   hours

The new  bus station  is a chaotic entry to Siem Reap. Please be patient if you have pre arranged a pick up.   Parthenon, Greece symbol by Greece-Athens.com   has a useful forum on this subject which makes interesting reading

 WARNING ABOUT BUS PACKAGES FROM BANGKOK Almost everyone who uses Bangkok Tour Agency Buses have had problems; from overpriced visas, to horrendous journeys up to 18-20   hours in cramped minivans or pickups   arriving in the early hours. Arriving   at the   guest houses late at night you will be pressurized into staying at their guest house.   They   will not be   helpful   with transport or sometimes   even allow phone calls to your chosen hotel or guest house.

Most TAT agencies in Bangkok   say the journey takes 8-10 hours via Poipet. Don't believe them! At best It will take 12 or more hours.   Some   operators are now sending people to remote border crossings; taking 18-20 hours on   arriving at 2am.

The Bus that takes you from Bangkok to the border is usually a good air con vehicle . Some agencies take people to a remote border at   Pallin (Daung ) It is here that most problems occur. You are likely to be   asked to pay $40   or more for visa.   You will certainly be delayed by 3 hours waiting for the transport (usually overcrowded minivans or pickup trucks ). you will have an uncomfortable journey on dirt roads via Battembang; with numerous long stops and at least one well rehearsed breakdown. it can take 18 hours in total

Please see experience of traveller below:

Its possible to get a bus from Bangkok (any of the travel agents on the Koh Sarn Rd). Dont believe them when they say it will take 10 hours, the roads from Bangkok to the border are fine but once you cross into Cambodia its basically dirt track. It will take about 15-18 hours depending on how many times you break down (almost always at a restaurant).

When booking make sure you cross at Poi Pet as the roads are much worse at Duang and they will rip you off when buying your Visa (I paid $40 when it only supposed to be $20)