Before you go abroad, it's a good idea to check up on the latest travel advisories from the US State Department. These advisories contain helpful information on a given country's crime rate, health facilities, areas of instability, and US consular information. As a general rule, the advisories always err on the side of caution: for example, the Cambodian advisory at the time of our trip warned that "U.S. citizens defer non-essential travel to Cambodia." After consulting with other recent visitors to Cambodia we concluded the country was safe enough as long as we took the proper precautions. You'll have to make your own judgment.
The easiest ways to access the travel advisories are to either to visit the travel advisory website or to subscribe to the travel advisory email list, which you can do by sending an email to firstname.lastname@example.org with the word subscribe in the body of your message.
If you plan to visit Southeast Asia (or practically anywhere else), you better have your passport. US citizens can usually apply for one at a local post office. Visit the US State Department Travel website for more information.
Each country in Southeast Asia has individual rules for obtaining entry visas - even if you have a passport, you'll also need to get a visa to enter many countries. Visa rules often differ among nationalities, so you should check with each country's embassy or consulate in your home country. Visa policies may change unexpectedly, but at the time of our trip (November 1997), these were the regulations pertaining to US citizens:
Thailand. No visa is necessary for stays of less than 30 days. If your 30 days are about to run out, you can always leave the country and come back - you'll get a new 30 day allowance each time you enter Thailand.
Cambodia. Visas are available upon arrival at Phnom Penh's Pochentong Airport for $20 dollars. They'll also need a passport photo of yourself. If you plan to enter Cambodia by land, you'll need to get your visa ahead of time from a Cambodian embassy. As of 1998, it's possible to get a visa upon arrival at Siem Reap airport for flights originating from Bangkok.
Laos. Until very recently, acquiring a Lao visa was a painful process, requiring the assistance of a Lao-supported travel agency. You also couldn't get your visa at the border. As of mid 1997, rules have relaxed. At the time of our trip you could get your visa upon arrival at Wattay Airport in Vientiane as well as the Chiang Kong/Huay Xai and Nong Khai/Vientiane border crossings. Officially, this visa would require $50 cash (crisp bills only!), proof of a hotel reservation within Laos, proof of an onward ticket and significant funds. In practice, though, most travelers we met had no problem getting in without a reservation, onward ticket, or having to pull out all of their traveler's cheques. If you're worried about problems at the border, you can always get your visa in Bangkok or in the Thai border towns of Nong Khai and Chiang Khong, but again, you probably won't have a problem at the border as long as you don't act like a drugged out hippie or an evangelist with a bag full of bibles. Once inside Laos, you'll also be expected to register with the local immigration office for each city you visit (remember, though you may easily forget it while you're there, Laos is still a communist country). This is usually a breeze, since many immigration offices are located near airport arrival gates or boat docks (if you're using the Mekong for transportation).
Burma (Myanmar). If you plan to cross into Burma for a day trip from Thailand, you can get a one-day visa for five dollars at the border post. Officially, the border crossing at Mae Sai, Thailand requires you to leave your passport on the Thai side while you bring a xerox copy to the Burmese side, but when we made the crossing, immigration let us keep our passports and xeroxes. If you plan to fly into Burma and stay for awhile, you'll have to get your visa ahead of time either in the US or at another Burmese embassy. Prices for visas vary greatly depending on where you get it. Once inside Burma, though, you'll be expected to exchange $300 into Foreign Exchange Certificates (FECs), the local currency used by foreign visitors. This may be an annoyance, but once you're on your way it's usually easy to exchange your FECs back dollars or into Burmese kyats (the local currency) if you so desire.
Vietnam. Visas are available upon arrival at Hanoi and Ho Chi Minh airports. If you plan to enter by land, you'll have to get it ahead of time. Be sure to specify your entry and exit points, especially if you enter by land.