I live and work in NYC, which actually for such a large city has a small homeless population (we have 'em, but per capita we have less then many other cities... maybe we have better resources to get them help? I dunno).
I have over the course of my life learned a sort of sixth sense about who to respond to and who to ignore. I only give money to people trying to earn it, however pathetic the result (playing music, singing, dancing, telling jokes, doing magic, etc), straight up begging or making long speeches results in nothing from me, except occasional acknowledgement.
There are a few regular homeless people I see in the course of daily goings. Some I just immediately got a vibe from that it was ok to say "sorry" to as I passed - and I've been proven right. One older man in a wheelchair who begs outside of Starbucks and is routinely ignored by people. I walk by him everyday (I don't go in Starbucks its just on my path) and we now have gotten to the point of exchanging pleasantries - he says "good morning" or "stay warm" or "have a nice day" and I smile and reply in kind. I don't stop walking or even slow, but I do acknowledge him. And honestly? He seems happy just that someone is smiling and seeing him.
There are others though who I just get an aggressive, negative vibe from. Those people I just walk past with out even turning my head and certainly without responding.
And then there are some who I just sometimes find myself inadvertently giving dirty looks to. The couple that begs all around the neighborhood but who I see go into Starbucks daily. *I* can't afford Starbucks daily. There are coffee carts and delis on every street here, there are more affordable options! Or the young beggars who smoke. Cigarettes in NYC cost over $15 a pack, again, *I* can't afford to smoke, I certainly have no intention of supporting someone else's habit and actively resent being asked to do so.