Having worked for many years with the homeless and/or addicted, it is a terrible idea to "just give a little". It really is like the Southpark with the homeless people that moved into their town. Once they see you (or your area/neighborhood) as a source of income in ANY way, you're stuck with them until they find another sucker. The best thing to do is say no and move on. If everyone did this, they will find a real source of income (i.e. a job) or leave the area. I've heard all of the lines and tricks they use to get your change and it's truly sad. Most of them have so many stories it would blow your mind...
This is very prevalent around Christmas to garner sympathy, but giving money to people standing on corners with signs asking for help is NOT a good idea. You aren't helping them, unless it's to get high. At least in my city, the ones who want help GET HELP through programs that help them find housing and jobs to get them back on their feet. I've seen it work time and time again and I keep in touch with some of them.
Ever actually ask someone who has a sign that says "Will work for food" to do a job for you? Most of them will look at you crazy, or give excuses as to why they can't do that particular job. They play on the fact that most people don't want to "deal" with them and would rather give them money than associate with them long enough to have them work for them. The same goes for the "need $$ for food" signs. Try to give them some food. They don't want your food (unless it's something good!) they want your money.
I know we want to think that it helps and makes us feel better about ourselves in the process, but trust me, it's hurting more than it's helping.