@MikeMason-- I understand Khmer about 90% and I speak fairly well too. But I can't read or write.
I learned Khmer mainly from my husband who is from Cambodia. He has taught me a lot and we spend a lot of time with his family members so I have many opportunities to practice.
I thought about taking classes but they are hard to find. Learning from books or DVDs is always an option but I was told that these teach formal Khmer which is not how the natives speak.
If you can find classes that teach colloquial Khmer or better yet, if you can go to Cambodia and take lessons from a tutor there, that will be the best way to learn. Khmer is not a very hard language, but like all languages, it requires time, patience and practice.