You can get away with pretty much haggling anything in Cambodia. Restaurants, outdoor food stalls, even rates for guesthouses. It doesn't even matter if you lose your temper as myths of "saving face" doesn't really translate to money. The Khmer are notoriously quiet up to a point of no return. They do not lose face, they lose their temper. However, there are a few guidelines:
Many products, especially those not aimed at tourists, are fixed price, and while it is possible to get a minor discount if you ask, you cannot get things significantly cheaper than this. Many markets have the prices of goods painted on the walls (in Cambodian).
Try to stick to areas that aren't flooded with tourists may not work. In Cambodia where dining out isn't really common among local people, restaurants almost cater for foreigners and tend to be a little bit more expensive than neighbouring countries. However in Siem Reap, it is, sometimes if not always, possible to haggle with street food vendors over the portion of a dish, free side dish, and get 20-30% discount.
US dollar is widely used in Cambodia but no circulation of coins will end up giving you a lot of Cambodian Riels when the price you pay is not an integer. This gives a chance for shortchanging, which is particularly popular in several grocery stores in Siem Reap. For example, you give $1 for buying a bottle of water which is $0.6, the staff should return the amount of riels equivalent to $0.4, but they may keep some of them. The money cheated is usually minimal. Just be smart at mental arithmetic.
Haggle in groups. This is the key. Having two other friends will make it much easier to convince Cambodians to give a discount. One person can play bad cop, the other good cop.
Ask to speak with the manager/owner (this applies to guesthouse and restaurants). Usually if you try to haggle at a restaurant or guesthouse the employee will say that the boss needs to be there. If so, then just ask to speak with him or ask the employee to speak with him. You would be surprised at how easy it is to haggle down once you speak to the boss, many times he doesn't even want to be bothered and will give the discount to you.
Never pay the asking price for anything near the temples of Angkor. This includes books, souvenirs, paintings, water, and food. During the offseason, the foodstalls near the temples will have a separate menu, ask for it. You can even bargain on top of that too! Note that it's much harder to bargain at the foodstalls at Agnkor Wat and especially at the breakfast restaurants across the street from Angkor Wat.
Try not to haggle too harshly with the moto drivers and tuk tuks that work near where you stay. Most are honest, but they will look after your safety more if you are seen as a good customer. Some will decide they will get the money from you another way, and could take you to be mugged. Agree upon the fare before your ride or you may get into a very uncomfortable situation.
If haggling isn't your strong point the easiest way to get a good price at a market is to pick up an item, ask how much, look disappointed and start to walk away. The price will usually drop twice as you walk away with vendors unlikely to go below this second price.
Siem Reap is the easiest place to bargain, Phnom Penh may be a little harder but still worth a shot (worked at a guesthouse in Phnom Penh). Just remember to be persistent.