I never give money to homeless people, you think they can't buy drugs with one dollar, but if they get 100 people to think that way, then they can afford it. Or $5 will buy some cheap wine.
If you wanna give them something, like if they say they're starving, buy them a sandwich. I've bought McD's for people that asked for money. Don't forget that a lot of homeless people can get food at shelters and food kitchens, so if it REALLY bothers you, go donate some food there.