Cambodia is a tropical country. Therefore, take thin cotton clothes. For the evening, a high-necked T-shirt with long sleeves or a thin cotton sweater is handy as protection against mosquitoes. Take note that it is better to take too little rather than too much. Whatever you may miss you can often buy for a small sum. An umbrella is useful, both from the rain and the bright sunlight. A wool sweater or vest (especially in winter) is indispensable, especially in the mountains. Rainwear is useful during the monsoon. A few pairs of good, comfy walking shoes with good tread and a pair of slip on shoes is everything your feet will need. Remember that in the Buddhist shrines of the country one should enter barefoot.
Important items: sleeping bag liner, towel, sunglasses, sunscreen, toiletries, first aid kit, a hat, photographic or cinematographic equipment and enough film, extra batteries, flashlight, pocket knife (remember not to take on board the plane!), lighter, possibly a sustainable small stock of snacks and candy, clock, stationery, books, passport, sufficient funds, copies of passports, visas and travel insurance, a list with the numbers of any travellers cheques and insurance details showing the emergency contact number, your diary with important addresses, a good travel guide, booking docs of the trip.
Extra possibilities: mosquito net and rope to hang it, reading lamp, binoculars, map, games. For the local people you come into ocontact with, small items will be appreciated like postcards or pens. Cards are also very much in demand. You should pack your gear in a bag or backpack, not a suitcase, because it is difficult to transport. In the vans, the luggage can be transported on the roof, and since the roads in the country are very poor or unpaved, this can be dangerous if luggage is top heavy. In addition, a small backpack or shoulder bag is handy for the daily use. For safekeeping valuables, you should best buy a thin cotton money belt that you can wear under your clothes. Have a change of clothes in your hand luggage and items to keep you going for 48 hours, in case of baggage delay. For example: the pill, own medication, E.g malaria, and your toothbrush. Make sure your luggage is not too heavy! Very important (if you purchase the visa on arrival), two passport photos are required for the visa for Cambodia.