Cambodia is generally a very safe place for Ex-pats and tourists. If you follow some general guidelines you won't have any trouble. I think the most effective guideline is:
Don't do anything you wouldn't do in your own country.
Don't intentionally damage a car, this includes pissing on it when you're really drunk. The locals take great pride in their cars and have been know to shoot people in the legs and arse for willful damage. Generally it's the rich and powerful or their bodyguards that do this.
Don't expect people to drive reasonably or even sanely, the road rules here are flexible. Be aware of vehicles driving on the wrong side of the road without their headlights on at night by a person that's watching TV, talking on the phone and drinking beer and talking to the 20 other people in the car. Don't worry, the 5 year old stearing the car isn't drunk. Don't you just wish that was a joke!
Avoid putting your feet up on the bar, it's just rude and will probably get you throne out of even the quietest of bars.
Avoid getting into fights with the locals, they don't fight fair, have a pack mentality and will only come back with more friends if you do beat them up.
Only carry enough money for your needs that day, don't tell people how much money you have. It's better to suck up the charges using an ATM to withdraw money than to have it lying around in your in room safe. I personally know of two people that have lost money, one from an in room hotel safe the other from leaving the money in his girlfriends house for one day.
Don't leave your cell phone on the table when you are eating at a restaurant or near a window, even when you are there. A common way of stealing stuff is to go fishing with a long pole.