1. Before you start out
Do hire a guide if you want to get in-depth historical and cultural information and have access to the best routes in which to explore each temple. The guides also know where all the good carvings/reliefs are and can show you those as well.The guide will also be in charge of arranging transportation, so do specify if you'd like to go by tuk-tuk, car or mini-van.
If you arrive in Siem Reap in the late afternoon, do go and purchase your pass at 5:00pm. They will let you into the compound and you can catch a sunset (the sunset shots are great at Angkor Wat westside). Your pass will be activated the next day.
Do specify what kind of Angkor Wat pass you want. The normal passes are good for 1 day ($20), 2 or 3 days ($40) and 1 week ($60), however you can get the 3-day pass to be used within one week, and the 1-week pass to be used within one month. Don't forget to specify this at the time of purchase.
Do keep your pass handy, as you will be asked to show it when entering the temples. A plastic pass case on a leash comes in handy, and will protect the pass from getting soiled or damaged.
Do go at your own pace. It will prevent you from getting heat exhaustion. If you need to sit down and have a cold drink, just let your driver know. Try to go away from the crowds (you may be surprised to find something special), you can always follow them later.
2. Money and tipping
Do bring small bills ( US$1 and $5) for shopping and tipping. US dollar becomes unofficial currency in Siem Reap and many vendors, even street vendors prefer US dollar. Vendors simply price their products in US dollar but most market vendors are not going to have change for a $50 bill.
Don't change US dollar to the local currency, the riel at a bank and you will lose about 9 percent since bank exchange rate is at 1 USD= 3700 riels while vendors are pricing at 4000 riels. If you receive large bills from bank like 10000 or 50000 riels, it will even make more difficult for tipping. Riels in small face value is good to have and convenient for tipping, and for donations when visiting the temples.
Do tip for outstanding service. This includes guides and drivers ( roughly $2~$3 per day, each), masseuses, and waitstaff at restaurants. For waitstaff, "rounding off the bill" by leaving some bills (riel) will be fine. Do know that tipping is not expected, but will be very much appreciated, and whether to tip or not and the amount to be given is entirely up to the discretion of the visitor.
Do seriously consider what you will be doing before you purchase items from children at the temple. If they are making money there, why should they bother to attend school? Additionally, the books they are selling are cheap copies and the authors do not get royalties from the sales. Sharing a smile or a chat with these children is a better option, and at least they will get to practice their English with a friendly visitor.
Do wear lightweight garments. While many people opt for airy cotton fabrics, others prefer synthetics such as the kind that are available at mountaineering stores. The climate is very humid most of the time, and the synthetic fibers wick away sweat and dry on your body.
Don't wear revealing clothing- short shorts or midriff-baring tops, low cut blouses, etc (especially for women). Longer shorts, like right above the knee or capri pants are perfectly acceptable. Do note that if you wish to climb up to the towers atop Angkor Wat that both men and women should be "respectfully dressed", i.e. covered shoulders and chest, and long pants or skirts (long shorts are acceptable).
Do bring a hat or cap, and sunglasses.
Do wear sensible shoes. While technically the temples can be done in a pair of flip-flops, it would be a wiser choice to wear a pair of hiking sandals such as Teva or Keens. (Toe coverage is also a good idea.) While sneakers and socks or hiking boots may be overkill taking into consideration the heat and humidity of Cambodia, the bottom line is to wear what you feel comfortable in.
Don't forget a light jacket (such as microfleece) for early mornings, especially if you are going out by tuk-tuk or motodup. It tends to get very chilly in the wee hours with the wind blowing on you, especially in the cooler months.
4. Safety and health
Do take out an insurance policy that will cover medical treatment or accidents before you leave, or check with your insurance company at home to see whether you'll be covered when abroad.
Do use sunscreen. You can bring your favorite brand from home, or purchase it when there.
Do use insect repellent. While the risk of contracting malaria is very low in the immediate Siem Reap area, if you will be venturing out to the countryside (Phnom Kulen, Koh Ker, etc) then you will need to use a good DEET-based insect repellent. Repellent with a concentration of 25% is sufficient, and should be applied every 6 hours, or more frequently when you sweat. Remember, the sunscreen goes on first, the bug spray last.
Don't forget to drink plenty of water. If you are not used to the humidity (or dry weather), you may easily get dehydrated. You can check with your driver beforehand to see if he supplies water (many do, and even have a little cooler). if not, bring one bottle with you and you can buy more en route.