To reach Aranyaprathet from elsewhere in Thailand, see the Aranyaprathet article. The border crossing opens at 07:00 and closes at 20:00 there's no time difference between Cambodia and Thailand.
The easiest way to get to the border in Aranyaprathet is on the songthaew that also acts a shuttle to the Tesco/Lotus. It costs 15 baht, and they drop you directly at the real border crossing. Be warned if you take a tuk-tuk to the border: he will likely take you to the Cambodian Consulate to get a visa first. He does this, of course, because he receives a portion of the corruption fee for your overpriced visa 1000 - 1300 baht. Do not waste your time here and tell the driver that you want to go to the border instead.
Once you do finally persuade a tuk-tuk to take you to the border he will likely drop you off just before the border, at a group of "visa officials" wearing fake laminated badges. These touts will tell you that you have to get a visa for 1000 baht before entering Cambodia. This is, of course, a scam as you can get a visa on arrival after passing Thai immigration.
In the market area before the border there are a number of banks that offer decent USD exchange rates. However they don't open until 10:00AM, and after a weekend or holiday they may be short on USD.
Cambodian visas on arrival are available with varying degrees of hassle:
Arrive early in the morning to avoid queues, particularly at mid-day, when the tourist buses arrive.
The paperwork is very simple to fill out, and requires no assistance, regardless of what any touts may tell you.
Forms are available at the counter to the left of the visa window, although it's likely that a tout, seeking to establish a relationship for later, will bring one to you as soon as you approach the office. These touts are probably connected with the border officials and they will reassure to you, if asked, about the official role of the tout.
A passport photo is required for the visa. A 100 baht fee applies if you don't have a photo.
That's the easy part. The hard part sometimes is the cost. A sign posted by the Cambodian government over the window of the visa counter states clearly and unambiguously that a tourist visa costs US$20, however immigration officers have been known to ask for 1000 baht instead about $30 USD, or $20 with an additional 100-300 baht fee. If you agree to any of those charges, you'll likely have your visa within five minutes. If you hold to the price on the sign, though, you may be in for a long wait. This is another reason to arrive at the border early in the morning as in the late afternoon the official knows you want to get to your onward destination that same day, and therefore he has the leverage. If you arrive early in the morning, are polite, and learn to say hello/thank-you in Khmer upon walking into the office, you will find no problems and expect to get in without paying any sort of bribe.
In general, however, issues with immigration officials and visas are less pronounced than they once were. Just be aware that $20 +100 baht for no photo is all you need to pay to cross into Poipet, and immigration will eventually let you through.
Alternatively, you can obtain an e-Visa for US$25. This is an excellent service and you'll breeze past immigration.
Once you have your visa, brush off the touts and head down the street to get an entry stamp into Cambodia. Compared to the visa, this is a relatively straightforward procedure.
Free buses or shuttle vans should be waiting around the corner from the entry office. These travel to a transportation depot about 1km away, and deliver you into the hands of the Poipet travel monopoly. Tourists are no longer able to negotiate directly with drivers - the drivers know this, and travelers have reported being followed around Poipet by police officers to ensure they aren't able to strike up any deals away from the eye of the travel monopoly, who get a cut from every fare.
Anyone suggesting that you should change your money to riel the local currency is trying to get you to a counter with terribe exchange rates, perhaps around 3500 riel to $1, when the actual rate is approx 4100 riel to $1 Dec 2010. In any case, riel are only used as small change and the de facto currency in Cambodia is USD. There are plenty of ATMs dispensing dollars in Siem Reap.
Some tourists have reported they have been taken directly to a private travel agency instead of the proper depot, with the excuse that the station "is under construction". You can still refuse this obligation and walk away to negotiate directly to taxi drivers around.
Another chance to evade the monopoly is simply to walk left or right after customs and say you want to spend the night in a hostel to visit casino or disco. Go inside one of the hotels and ask if they can arrange a taxi. A taxi to Siem Riep normaly cost up to $30 USD, but without a proper negotiation you might be asked as much as $50-60 USD... good luck.
Past scams here have included having to pay for a SARS form or for non-production of a vaccination certificate.
By plane
Poipet's airport has no scheduled flights. If you're travelling from Bangkok to Siem Reap or vice-versa the alternative is to fly over Poipet - see the "By plane" section of the Siem Reap article.
By bus
Seats on the official bus service from the depot to Siem Reap cost $9 a pop, the ride will take around three hours, and you'll probably be delivered to a commission-paying guesthouse for a fresh round of hassles.
By Pickup Truck
If you can find them - they're persona non grata near the border these days - pickup trucks connect from Siem Reap and Battambang, although you will more likely than not have to change at Sisophon. Seats inside/outside the truck 5000/3000 riel to Sisophon, plus 10000/5000 onward to either Siem Reap or Battambang.