As with any place you are visiting, it's important to be respectful of the culture and the location. For visitors to Angkor, making sure your knees are covered will ensure you can even gain entry into the park in the first place. Make sure you're not touching artifacts or climbing on things you shouldn't be. It's best not to be loud, and it's always a good thing to ask a monk if you may take their photo first before shooting away. You are a guest in Cambodia and should act accordingly. In doing this, you can ensure the experience is one to remember.