Decide if you are going to Cambodia, or attaching Angkor Wat to a visit to another Southeast Asian country. Angkor Archaeological Park is near the town of Siem Reap, which has an international airport. In the nearby area, you can fly there from Laos, Thailand, Vietnam, Malaysia, and Singapore. These are all close enough that a run to Angkor can be added. If you are planning a vacation in Cambodia, generally, you are likely making Angkor your main priority and adding the rest of Cambodia on after that.
Take those anti-malarials. Angkor sits in a tropical forest, there is malaria around, and while you aren't likely to catch it, better safe than sorry.
Bring suitable clothing and sunblock. It is hot out there, and touring Angkor will mean being out in the heat all day, and the sun for part of it. Wear suitable clothing, bring your sunblock as well as your bug repellent, and don't forget the water.
Engage a tuk tuk (motorcycle with passenger trailer) driver or rent a bicycle. The hardy should consider exploring the Angkor ruins by bicycle. It is a pleasant way of getting around if you don't mind pedaling in the heat. Everyone else should find and negotiate for the services of a tuk tuk driver. These guys are not guides, but they do drive you from site to site. It is worth it to work out a fee to rent the guy and his vehicle for the entire day, because that way you can retreat back to Siem Reap for lunch, beat the mid-day heat, and also have a ride to dinner ready to go at the end of the day. The fee for that sort of thing should be about $25-30/day (2008). They are easy to arrange and are your best option for balancing the convenience of setting your own agenda against cost.
Get your own guidebook so you won't need a guide to each individual ruin.
Develop a thick skin. Everywhere you go in the Angkor ruins, you will be confronted by louts and scam artists, and cute kids trying to guilt you into buying something from them. Don't have a hard heart about this, but at the same time don't let your exposure to Cambodia's poverty take all the joy out of your visit.
Don't buy your park pass from anyone except the park authorities. Scammers and counterfeiters are out there.
Decide how much time you want to spend in the ruins. Park passes are available for 1 day ($20), 3 days ($40), and one week ($60). The one-day pass should never be considered: there is simply too much to see, and if you don't have the time to spend at least two days at the park, you should not bother going. 2-3 days will allow you to see all the major and some of the minor ruins. A week is for serious archaeological buffs, or people intent on sketching or painting things. The following major sites should be on your itinerary: Angkor Wat, the Bayon, Ta Prohm, Angkor Thom, and sunset from Phnom Bakheng. Try to spread them out, mixed in with smaller attractions, to avoid developing "it's just another cathedral" syndrome. Get an early start on each day. Getting to Angkor Wat at 8am means being ahead of the tour buses by a full hour, and once you are out in front of the crowds you can try to stay there.
