Travel guides: buy Lonely Planet or similar books as dead trees, not Kindle which is almost useless for this. Buy them in the US if it's convenient, because they definitely aren't cheaper in many other places.
Visas: US passport is usually good, but may increase the odds of being asked to pay someone off. In Cambodia specifically, better to get a visa in advance via the official website; some of the border stations are well-known for various schemes to get extra cash from those seeking visas on arrival. Make sure your passport starts out fairly empty--some visas take a whole page each. For US passports, extra pages are free initially, expensive to add later.
Room and board: if breakfast is free, great. But don't buy it in advance if it's not included, unless you're in the middle of nowhere.
Transportation: hitchhiking can be easier in poorer areas where many people do it, harder in places where everyone owns a car. For regional flights not every country does things online, and some smaller airlines do not charge a lot for last-minute tickets.
Packing: a 50 liter pack is enough if you don't have many gadgets. Take thin clothes that dry quickly, avoid jeans, wash a few items in the sink each night and enjoy perpetual clean clothes for free. An ultralight daypack like Marmot's Kompressor ($35) is useful, or get a cheapie abroad, e.g. $7 in Bangkok.