Don't plan a trip for only two days.
Do spend some time volunteering.
Don't hassle with making it to the temples for sunset and sunrise.
Do go out 2-3 hours before sunset to photograph the temples.
Don't bother over-packing.
Do bring flip-flops to slip off before entering restaurants/shops.
Don't stress about not knowing Khmer.
Do eat Khmer food.
Don't forget deoderant, headbands and extra contact lenses.
Do take advantage of a clean bathroom.

Don't buy gems unless you really know what you're doing.
Do get a tuk-tuk.
Don't spend all of your time in it.
Do bring small gifts for the children.
Don't give them money.
Do talk to everyone - local, tourist, traveler, volunteer.
Don't change your US dollars in for Cambodian Riels.
Do go to the Artisan Angkor workshop to understand the effort behind your souvenirs.
Don't miss The Soup Dragon's vegetarian spring rolls.
Do take a guide with you to the temples.
Don't forget to save US$25 for the airport departure fee.