Avoid big cities
As mentioned above, big cities in Cambodia are not a hit with the tourists. Of course, you cannot avoid them entirely. After all, you need to land somewhere. Also, Angkor Wat, one of Cambodia's icons, is located in Siem Reap, so avoiding it is not quite possible. Why should you avoid it? The answer is simple. First of all, the cities are not that interesting. It may sound harsh, but it is true. At the same time, you will not enjoy it. For example, Phnom Penh has been described several times as a city where there are no sidewalks, and you need to pay a tuk-tuk to take you from point A to point B. It is not very clean either, and tuk-tuk drivers will often tell you to keep your purse and your phone close so that petty thieves do not steal them. Comparing the cities with the natural parks... it is like you are in two different countries.
Do not forget bug repellents
Since you will be in the middle of nature most of the times, it is best if you pack some bug repellent. If you go to Virachey Natural Park for some trekking, there will be bugs. If you want to go way south to the beaches of Gulf of Thailand, you will encounter a lot of sand flies, and if you have ever been bitten by those, you know how much you want to avoid them.
Stay away from humidity
The natural parks tend to be more humid than other places, which is why you need to take that into consideration. You may think that it is no big deal, but in fact, you can have some real issues if you ignore it. Let's say that you go trekking into a natural park and the humidity levels are as high as they can be. If you do not pack extra pairs of socks, you may end up with foot fungus. It may sound gross, but it is a fact, and you would do best to avoid it.
Get your documentation online
If you are not from a visa-exempt country, you need a visa to get into Cambodia. In case you are eligible, you can apply for a Cambodia visa, which is an electronic version of the one you get at the embassy. You do not need much; only a passport and a few other things. If you are interested, even the passport photos can be acquired online. The visa will arrive in maximum four business days, and it will save you a lot of trouble.
The tips above are easy to follow, and it is enough to ignore one of them to make your trip miserable. Do your research and be prepared. It's the best advice anyone can give you.