Luggage and bags:
Backpacks and messenger bags are very convenient. Money belts also come in handy. You can't really tell it's there unless you're carrying a lot of money of course.
Clothing/Shoes/Weather Gear: The best type of clothes to bring would be shorts, cropped pants, t-shirts, and tank tops. For the rainy season, you should also pack a poncho. It keeps the water out and is very lightweight. Swimsuits and swimming trunks are great for the pool/beach. For the feet, flip flops are good because you can put it on and take if off real quick. Running shoes are also great for visiting ruins. I wore my flip flops 99% of the time I was in Cambodia.
Toiletries and Medical Supplies: I highly recommend bringing a lot of toilet paper because most of the public restrooms do not carry it. Carry a roll everywhere you go. Wet wipes or baby wipes are really good too. Also bring with you; pain reliever/fever reducer, Immodium, and a mini first aid kit.
Photo Equipment: Digital cameras! If you love to take photos of everything and anything, carry a 4G memory card or two 2G memory cards. Depending on the resolution, you could get over 2000 photos with a 2G SD card. Tripods are a must. Especially if you want to get everyone in the picture and there is nothing the camera can rest on.
Luggage and bags:
Traveling light is imperative if you want to do things like take the river trip from Battambang to Siem Reap. A backpack is sturdy and more easily taken onto and off the boat via the wooden planks provided.
Clothing/Shoes/Weather Gear: Sandals are nice but sturdy boots are better for going from point A to B with a heavy pack.
Photo Equipment: A wide angle lets you capture the vast expanse of river life but a good zoom is essential for capturing locals going about their daily chores.