Oh btw Katie if you're travelling between Sihanoukville/Kampot and Phnom Penh the train is, as mentioned, now an option but it only runs on weekends.
I caught it from Kampot to PP in November and it is most definitely the best way to travel that route. It passes through some stunning countryside, right through rice paddies etc...far preferable to the hot, stuffy and dangerous bus journey up NH1
Takes only slightly longer than a bus too, around four hours. To book tickets go directly to the train station yourself.