Overland from Siem Reap to Bangkok
Buy bus or minivan tickets from the numerous travel agents in Siem Reap. Be sure to haggle and shop around a bit. Buses leave from the main bus station whereas minivans pick you up from your hotel. Both leave early in the morning (7-8am) so buy tickets at least the day before. You can buy tickets to either Poipet (the border) or straight through to Bangkok. See below for pros and cons of each.
You don't travel the whole journey in the same vehicle so you'll be dropped off at the border and your bags unloaded. If you've bought a through ticket to Bangkok someone will check your ticket when you arrive and give you a sticker to identiy your onward travel in Thailand.
Leaving Cambodia is easy and takes 5-15mins. Thai immigration is a few meters down the road on the left. Expect to wait here for several hours (seriously). Typical times from joining the Thai immigration queue to entering Thailand range from 2 to 4+ hours. If the queue is long you'll be waiting outside in full sun for part of the time.
Once inside, unless you hold a Thai passport or need to pay for a Thai visa, go for the immigration queue second from the right. The furthest right desk is for paid visas but there aren't many of these so when there's no queue they call others over from the nearest line so it can be significantly faster.
In Thailand, you can get to Bangkok via taxi, private minivan, private coach, public bus or train. If you want to take the public bus or train you'll need to get a tuk-tuk or taxi to Aranyaprathet, about 6km from the border. The public bus is comfortable but quite slow as it stops at many towns en-route to Bangkok. The train is also slow and there are only two a day; depending upon your border crossing experience you may miss the last train.
The fastest way is via private road transport: taxi, minivan or coach. Taxis are naturally the most expensive. Minivans and coaches are good value but they don't leave until full. Bear in mind that even if you arrived in a full minivan from Siem Reap, some passengers may be going to other Thai destinations meaning you'll have to wait for others before departing. If you're one of the first over the border you may have to wait up to 2 hours for the full complement of through-ticket passengers; but if you're one of the last then it would only be a few minutes. The through ticket option is only a couple of dollars more than a ticket to Poipet so is good value.
Once on the road to Bangkok, the Thai authorities may well check a few times for people trafficking. This may involve stopping in a layby where armed soldiers look in the van/coach, which can be a bit alarming at first!
The road to Bangkok gets very busy as you approach Bangkok. It may take a few hours from reaching the edge of Bangkok to being dropped off at your final destination due to the traffic. Don't worry if you don't arrive close to your Bangkok hotel; Bangkok taxis are very cheap and plentiful - although do ask the driver to use the meter and get out if they refuse.
Door-to-door, expect the whole journey to take from 8 to 12 hours.
Summary
There's no cheap fast way to travel from Siem Reap to Bangkok. A through ticket on a minivan purchased in Siem Reap is probably the best option but expect a general lack of information on your journey and delays at the border and in Bangkok traffic.
UPDATE: July 2013 - A faster, easier, but not cheaper way
The new direct Bangkok-SiemReap-Bangkok route by the Thai Government-run Transport Co., means there is now an easier, faster, though possibly more expensive way of doing did this route.
The fare is $28, and tickets are sold from Nattakan Office near to the KFC on the main Sivartha Road. (User recommendations are to purchase a seat closer to the front to avoid any possible unsavoury smells from the toilet in the rear of the bus)
A Tuk Tuk will collect you from your guesthouse any time after 7 to bring you to the Nattakan office, from where the bus leaves around 8-8.30am. The journey takes about 7-8 hours, arriving at the border around 10.30/11am, where you are given "if lost please contact" tags and helped through the Cambodian exit point and Thailand entry point. At this latter point you will be instructed to take your luggage off so it can be scanned just after you pass immigration. However you then get on the same bus to make the rest of the journey to Bangkok
They provide a 'refreshment' in the morning after your board, a small bread-type thing, and a cup which they'll fill from a coffee flask should you wish. After passing Thai immigration, there is a stop where you are given a boxed lunch (rice with omelette) on board. There is no opportunity to get anything else. It may be an idea to plan ahead and bring lunch with you.
Arrival at Mo Chit is estimated at 4pm, however it can be as early as 3.15. From there go around to the right of the complex where there are lots of metered taxis - be clear: "Meter on - ka/kap". Or if you are solo, grab a orange/pink vested moto-driver, who will bring you to the Mo Chit BTS station for 50THB. From there you get make your way to train station, airport, or into town. If you are going to Kao San, you might as well get the taxi to bring you there.
http://www.tripadvisor.com/Travel-g2973 ... ngkok.html