Siem Reap in February was very hot --- so when choosing what to wear, light, loose fitting pieces should be on the top of your list, especially those made with fabrics that have moisture-wicking properties.
Old Navy cargo pants
aviator shades in silver
Kalso earth shoe sandals in almond
Cambodians are by nature modest, so although the temptation is to dress with less, choose items that will cover shoulders and reach at least the knees. The temples of Angkor Wat in Siem Reap are sacred, even if not currently in use for worship. Another benefit to being covered is to have less surface area vulnerable to mosquito bites. Malaria, dengue fever, and typhoid are souvenirs you don't want to take home from your trip.
Templing (a verb that seems only to make sense in Siem Reap) involves ruins, climbing over fallen architecture or large stones so cute little flip-flops are not a good idea. Durable, practical sandals with good support or some lightweight sneakers are footwear of choice when exploring the temples. A good hat with a wide enough brim is also necessary protection against the searing sun.
I brought insect repellent spray and also travel wipes with at least 30% deet. Apply your sunscreen first then use your spray or wipe. According to my doctor, all you need is 30% deet for any insect repellent to be effective. Less chemicals, the better. However, I really didn't want to take a chance with any of the natural insect repellents because I have heard too many accounts of them being useless. A word of caution: I used the insect repellent wipes (used above) on the go and was horrified to find that my nail polish came off so either use carefully or wear your nails au naturel :)
Another precaution you can take is to spray your clothes, including any hats and backpacks, with permetherin prior to your trip. Spray each side at least 30 seconds, let dry for 2 hours and these items will be good to go for up to six washings (as well as if you sweat heavily throughout the day). It is completely odorless when it dries and does not ruin the fabric in anyway.