Although it may seem like you are
doing the right thing by giving change to the
homeless, you really may be causing harm by doing so.
Many of the homeless are abusers of drugs or alcohol and would quickly take any money given to them and put it toward their habit. Not ALL the homeless, but many, and it can be difficult to tell the difference.
If you want to do something to help, donate your time and/or money to the shelters that help the homeless by providing food, shelter and trying to help them get jobs. Become familiar with where your local shelters are and direct the homeless person there when asked for change.
Or, if you really feel that you need to give something right then, offer to buy a meal for the person. At least that way you know that you have made a positive investment in this person.