While this is a question that is best answered by Cambodian PCVs, I'll weigh in.
My main suggestion would be to make sure you can carry everything you bring all at once. Expanding duffel bags are great, the ones that you sling over your shoulder. They can also then be packed down and stashed pretty easy.
Many people ask about rollers, and depending on the country they're really only useful in airports. I would suggest a backpack style, too. One that you can use when travelling out of site, carry for longer durations, fits between your feet/legs when on a bus, and can handle being tossed around and other rugged things. If you want, a run to REI or similiar outdoor store would help; tell them what you're doing and they should have some recommendations. Top 2 important things are that it fits comfortably and that it can handle being tossed around. Size and style are personal choices.