According to your level of satisfaction
If you enjoyed your massage, a cannot be got round activity in Cambodia, you can leave a tip to the masseur.
If you take a private driver, whether he/she drives a tuk-tuk or a car, it is well appreciated if you give a 2 or 3 dollars tip aside from the initial price of the service.
Local people do not give tips as it is not the custom. The amount you give depends on your budget and your generosity but in every instance, those you receive a tip are always grateful as the standard of living is very low in Cambodia.
At the restaurant
Gratuities in restaurants will depend on your level of satisfaction, budget and on the place you decide to eat. People won’t expect any tips in local and small restaurants but they will be more likely expected in prestigious restaurants.
While traveling in Cambodia, if you decide to take a guide for a day or more, tips are highly appreciated at the end. Especially if he/she should speak perfect English, was flexible according to your desires and showed you some unique places. You should consider giving them 5 dollars per person for the entire service but as we already mentioned, nothing is mandatory.