Reddit mentions of Dos & Don'ts in Cambodia

If you have the time, read Dos and Don'ts in Cambodia. It's a great answer to your question.

1. Cambodian adults dress more conservatively than adults in the west. Shorts are not common dress. Wear pants if you want to fit in.
   
2. Don't point at or touch others with your feet. This is very disrespectful. You probably won't be in a position to, but it could be easy to do by accident at dinner time because it's customary to take off your shoes and sit on the floor together for meals.
   
3. Don't pat or rub people's heads, especially elder people. This is considered rude. not as rude as pointing with your feet, but close.
   
4. Age is important in Cambodia. If someone's older then you, even by a year or two, it's customary to show respect.
   
5. How you address a person depends their age. E.G. For males, if someone is your peer your call them bong (brother). If they're ~10+ years older than you, call them "poo" (uncle). If they can be your dad or your dad's dad there's another name you should address them as, but I forget.
   
6. In general Cambodia is conservative country. When in public (outside of the nighclub areas) act like you're walking around downtown in a bible belt city, not like you're in Vegas. Don't express anger, be vulgar, drunk, or loud in public.
   
7. Saving face is important in Cambodian culture. Smile, be polite, and don't start conflicts.
   
   All that being said, Cambodia is changing rapidly. These traditions are not as strict as they were in the past, especially in Phnom Penh. Just don't be a jackass and you'll be fine.