* Fly in, don't drive it. If you must drive in, check out the Wikitravel article about whichever border crossing you choose.
* Get your visa in advance, you might not get overcharged if you do it from your home country.
* General advice for travel: bring tissues, handi wipes, some snacks & chewable pepto tablets. You will not regret it.
* Spend a full week in the country if at all possible. We rushed and probably missed a lot of good stuff (Mekong river tours, elephant excursions etc. etc.)
* Wander around & eat at least one meal in a hole-in-the-wall restaurant where actual Cambodians are eating. (Make sure you have the pepto tablets with you!)
* Do your research in advance. This is the number one thing I think we failed at. We planned everything kind of at the last minute, and although we did some research...more would have been better. Read up on Angkor Wat so you know which temples you want to spend the most time at. Study up on some Hindu imagery so you'll have an idea of what you're looking at. Also, decide in advance if/when you want to explore the Killing Fields / Khmer Rouge aspects of Cambodia history. I felt a little guilty for avoiding all of it. Make this decision in advance & schedule it in (or not). But it sucked to kind of have that hanging over our heads "we should go..." and yet not do it.
* Just do it! For all the ups and downs, I'm glad I went. Cambodia is not for the faint of heart, but Angkor Wat is really amazing. If you've ever dreamed of going... just do it!