2 experiences from my mission, and some training I received as a manager, swayed me to not give to random "homeless" people. I give to organizations, not individuals, where I think the most good can happen.
So the first experience was a homeless man asking for money in Germany while I was a missionary. I asked him what the money was for. He said food. We took him to a local grocery store and let him pick out the food he wanted, then bought it and gave him the bag. As we left, I glanced back in time to catch him staring at the bag, then watched him dump the entire bag and all its contents into a garbage can.
The second experience was during a street display (we did a bunch of these on my mission, they could be fun). There was a woman there on a blanket with a small child begging when we arrived to set up (about 8 am I would guess). We set up our display and then the other missionaries participating showed up and we spent the bulk of the day there, handing out BOMs and talking to people. I glanced over from time to time to see if people were giving her money, she looked very bedraggled and down on her luck for sure, and the kid looked pitiful. Occasionally she would get up and shift positions, looking for all the world like a full on cripple. All she ever had was a few coins on the blanket in front of her, but I did see several people drop coins there (usually 2 or 5 mark coins, so between a buck fifty and 4 bucks was normally what I saw people drop). One of the other missionaries gave her a 5 mark coin. At the end of the day, when the shops were mostly closed and we were taking down the street display, I saw her pack up her things. She stood up, revealing a fairly full bag of coins from among her blankets and coat and such, and a shiny black very new model mercedes pulled up nearby. A young guy in what looked like a very expensive suit with shiny shoes and shinier hair hustled out and helped her pick up her things. She walked, with no sign of pain or any kind of malady suddenly, with the child to the car, got in the back seat and they sped away.
The training I had was called Performance Management, and it was conducted by the Aubrey Daniels group. In short it focuses on reinforcement to drive behavior. The instructor got us into a discussion of panhandlers and what drives that behavior. In that discussion it became so clear what the drivers were behind panhandling and how reinforcing that behavior did nothing to actually help the individual, in fact panhandling can be very dangerous (the guy had statistics on hobo to hobo crime, death rates, all that kind of stuff, that was really bad, and showed that active panhandling is more dangerous than just living on the street and not actively participating in panhandling), and when we reinforce the behavior by giving them money it puts people at greater risk and causes the problem overall to get worse.
So I do not give to individual panhandlers or homeless people. I give to organizations that I feel will do something with the money to help the most people the best way possible.