I really feel I have to mention the fact that due to Cambodia’s recent brutal history, the number of landmine victims, amputees, and orphaned children is so omnipresent that you will be constantly confronted with them when moving around.
Yes, there are many beggars, but consider that you are in a country where the numbers of these victims is so great and the majority have no form of income in a country where no welfare system exists as a safety net.
I normally wouldn't condone giving to beggars, as most other countries I have been in have a large number of professionals that do beg for a living, but I dare even the most cold-hearted person not to be moved by some of the victims. One person that really sticks in my mind was that of a young guy who had lost both legs and one arm and whose only means of transport was lying facedown on his stomach on a skateboard – imagine moving through life like that? Think how much your small amount of riel means to him.
Before you brush them off, think of passing on a dollar when you have so much. I know that you can't give to every one of these, people but just don't ignore them as if they don't exist.
Many of these people do not want to rely on begging, so you will encounter many booksellers and the like. If you are looking at purchasing books on Cambodian culture, buy from these people. Not only will you save money, but you will also be helping them.
Also, another small way to help is to try to choose a restaurant, such as the Soup Dragon, that donates $0.07 from every dollar spent to the Royal Hospital for Children.