All roads to Aranyaprathet have checkpoints and most buses will be boarded at least once by uniformed military/immigration personnel looking primarily for illegal immigrants from Cambodia. Typically a small number of Thai/Khmer passengers will be asked to produce their ID cards; foreigners (ie non-Thai/non-Khmer, and especially Westerners) are not usually required to produce their passports.
There are reliable and inexpensive government bus services from and to many destinations, including Bangkok (frequent), Buriram (frequent), Chachoengsao (frequent), Chanthaburi (frequent), Mukdahan, Nakhon Ratchasima (Khorat) (frequent), Pattaya, Rayong, Surin, and Udon Thani. For Trat, go via Chanthaburi.
There are direct first class buses from/to Suvarnabhumi - departures from Suvarnabhumi are at 08:00 & 14:30, departures from Aranyaprathet are at 09:00 & 14:30. The fare is 187 baht.
The Suvarnabhumi Airport bus terminal can also be a good way to connect with other destinations, including Ekamai and many other points in Bangkok served by Suvarnabhumi's extensive and frequent "local" bus services, as well as the western seaboard of Eastern Thailand.
To connect with Don Muang, use the Moh Chit bus services. When going to Aranyaprathet, it may be possible to save time by boarding the bus at Rangsit; the catch with this strategy is that 1st class buses won't stop there if all seats are already taken. When going to Don Muang, request to disembark directly in front of the airport.
First class buses from/to Moh Chit run every 30-60 minutes and take 4-5 hours; the fare varies depending on the exact route, but is around 200 baht, which typically includes a small snack, a soft drink, and a bottle of water. Second class buses cost less but may take longer and don't have on-board toilets.
From Ekamai, take a bus to Chachoengsao then switch to an Aranyaprathet bus. This will probably be the quickest option if you're already at (or want to get to) Ekamai, but from/to elsewhere in Bangkok it's usually faster to go direct via Moh Chit. An alternative strategy for connecting with Ekamai is to go via the Suvarnabhumi Airport bus terminal - see times, above.
Khao San Road tourist buses are more expensive and less comfortable than government services; those to destinations in Cambodia always involve a change of vehicle at the border, and are usually full of irritating people.
Four trains a day connect Aranyaprathet with Bangkok's Hualamphong Train Station. A morning train departs Hualamphong at 05:55 and arrives at 11:20 (in the opposite direction 06:40 & 11:45); an afternoon train departs Hualamphong at 13:05 and arrives at 18:05 (in the opposite direction 13:40 & 19:05). The fare is 48 baht; the surcharge for a bicycle is 80 baht.
For those with a little time to kill, the train can be an enjoyable way to connect with both Suvarnabhumi and Don Muang airports. For Suvarnabhumi, go via Hua Takhe station and BMTA shuttle bus (15 baht); for Don Muang, simply switch trains at Hualamphong.
To reach Poipet from elsewhere in Cambodia, see the Poipet article. The border crossing opens at 07:00 and closes at 20:00. (There's no time difference between Cambodia and Thailand.)
Entering Thailand from Poipet is straightforward for travellers entitled to enter without a visa and for those who have obtained a visa in advance.
Nationalities entitled to visa-on-arrival facilities should note that visas are '''not''' available on arrival when entering Thailand overland from Cambodia, and should either obtain a visa in advance or else enter by air (there are visa-on-arrival facilities at all Thai international airports).
If you cross from Thailand to Cambodia in the morning, you're likely to see a long line of Thais waiting for an one-day exit stamp to gamble at the casinos in Poipet. However there's a separate line for non-Thais and in a welcome change from usual order of preference, it's quite a bit faster than the one the natives have to use - at least until mid-day, when the Khao San tourist buses arrive.
The bus terminal and the train station are very close to each other, and about 6km from the border crossing. A "local" bus service connects the three, if you're not in a hurry.
Tuk-tuks to the border cost 50-60 baht. Don't let them stop at travel agencies along the way to get your visa, tell them you want to go to the immigration office at the border. These travel agencies are a scam and will charge you an extra "service fee" above the US$20 cost of the visa and will delay your trip by hours. The tuk-tuk driver gets a commission for leaving you at the travel agency rather than taking you all the way to the border.
Motorcycle-taxis are also readily available, and charge less than tuk-tuks.
If you're looking for breakfast in the morning, you'll find most restaurants closed, but there are fruit sellers in the market, and some convenience stores. Numerous restaurants are open in the afternoon and evening.
If you're on your way to Siem Reap in the morning, get a good night's sleep, because you have a bumpy ride ahead of you.
There are more than 6 internet cafes in the center of (0.5 kilometer radius) Aranyaprathet. They have broadband and are reliably fast. 20baht/hour, or 40baht /3 hours.