As foreigners, most Khmer will not expect us to necessarily be familiar with their system of etiquette but will be impressed if it is demonstrated by us. Moreover, familiarity with the general principles can help avoid causing unintended offence.
When meeting, especially with social superiors, Cambodians traditionally Som Pas while saying Choum Reap Sur (Hello). When leaving, they will again Som Pas while saying Choum Reap Lir (Goodbye), usually as a display of respect. If you are greeted in this way, it would be impolite not to return a Som Pas as it would be tantamount to us rejecting an offered handshake in our culture.
However, Western cultural mores are increasing common and accepted in Cambodia today. Cambodian men often shake hands. Women meanwhile, are more likely to adhere to the traditional style of greeting and sometimes reluctant to shake hands – as touching in public, especially between the sexes, has never been the convention here.