I wish this guide posted on the Matador Network was available before I did this border crossing. Border Crossing Guide: Bangkok to Siem Reap
Here are a few of my own tips:
1. Do. Not. Buy. A. Direct. Bus. Ticket. From. Bangkok. To. Siem. Reap. Unless you like to spend way more money than everyone else. This “direct” bus drops you off at a travel agency in the Thai border town. Then they tell you that you have to pay more than what the visa costs because you didn’t buy it in advance, and then they take your passports and walk to the border crossing, get the visa for you, and pocket the extra cash. They will smile. They will be friendly and reassuring. All the other clueless travelers will exchange looks with each other trying to seek comfort in this. It is all a scam. Just ignore them and walk to the border crossing.
2. Before you pass through the crossing in Cambodia, your chaperone will tell you that the bus from the Cambodian border town to Siem Reap takes 6 hours, and it’s on a horrible road, and best of all, it won’t be ready until x (x is the time they figure is too long for you to wait). But you can use a share taxi! This, of course, isn’t included. It’s not recommended that you wait for the bus… because who knows when it will get there… and it takes so long to get to Siem Reap. I’m not sure if there is even a bus that exists. This article recommends you take the share taxi (and in my experience, it was $40 per taxi ride, 4 people in a taxi).
After you get to Siem Reap, and see the Temples of Angkor, it will all seem worth it :) Good luck!