If you're generally a respectful person, you shouldn't find any problem fitting in while visiting Cambodia, but there are a few cultural customs in this Southeast Asian country that are different from Western areas. They are fairly common, though, and even if you forget one or two, tourists' faux pas are for the most part forgiven.
Do: Take Your Shoes off at the Door
The feet are considered to be the dirtiest and least sacred part of the body. You will see almost every tourist and local in Cambodia wearing flip flops on a daily basis and that's because it's customary to slip your shoes off when entering a place—not just someone's house or a hostel. You'll be expected to take your shoes off at temples and many restaurants, too.
Don't point your feet at people, especially at images of Buddha, and don't allow people to see the bottoms of them. Even putting your feet on the seat opposite of you is a bad idea.
Don't: Interact With Monks
You're bound to see many a monk while traveling in Cambodia, so you need to know how to interact with them—or how not to interact with them, rather. Women, especially, should never touch a monk or hand anything to them (even the monk's mother may not hug her son while he is a monk).
Most Theravada monks are not allowed to eat after noon, so be mindful by not eating or snacking around them during this time. Likewise, if a monk is seated, you should sit also before starting a conversation. Try to sit lower than them if you can.
Lastly, do not touch a monk's—or anyone else's—head. It's a sign of disrespect.
Do: Eat Only With Your Right Hand
Business and eating are typically conducted with the right hand only; the left hand is reserved for dirty duties in the toilet. Avoid handing people things with your left hand and try to use your right hand exclusively when eating.
Don't: Flaunt the Fact That You're American
Be mindful of Cambodia's war-torn history by not bringing up sensitive subjects such as war, politics, violence, or the Khmer Rouge. Almost everyone in this country has lost family and friends to violence and Americans have been a big part of it, so be patient if they hold a grudge. Definitely avoid wearing T-shirts and clothing that depict war or violence.
Do: Speak the Local Language
Don't worry about the locals laughing at you for your poor language skills. Most of them appreciate your effort and help you through it. A lot of people don't even speak English, so always ask first.
The traditional Cambodian greeting—called a som pas—is made by putting your two hands together in a prayer-like gesture in front of the chest with fingertips pointing up. Give a slight bow with your head. This is the equivalent of the wai in Thailand.
You can thank them by saying "arkun." Most locals will greet each other with "hello."
Don't: Dress Too Skimpy
It's hot in Cambodia, but the temperature is no excuse for skimpy clothing. Modest dress is the rule, particularly for women. Although many tourists wear shorts, the locals tend to cover as much skin as possible.
Local men typically wear collared, short-sleeved shirts and long pants. Although wearing shorts and a T-shirt is fine for tourists, you should try not to cause locals to feel embarrassed by your attire. Avoid short shorts, miniskirts, tight yoga pants, or other clothing that is too revealing.
Although tourism has caused local dress to lax somewhat, always dress conservatively when visiting temples (that includes the Angkor sites), homes, or entering a government building. Avoid wearing T-shirts with religious themes (images of Buddha or Hindu deities). Cover your shoulders and wear pants or a long skirt.
Haggling prices is an uncomfortable and seemingly disrespectful activity for many Westerners, but it's expected here. When negotiating prices, allow the other party to save face by giving just a little on the final price. Alternatively, you can return to buy from them again later.
Don't: Show Affection in Public
Cambodians are conservative, which means they frown upon public displays of affection. Again, the key is to not cause someone to feel embarrassed. Holding hands is okay, but snuggling intimately on the bus may not be. Be mindful in your contact with the opposite sex; even placing an arm around a local to pose for a picture can be misinterpreted.