I live in San Francisco too and I have to say it's OK to just buy the dude a sandwich if you feel bad or go ahead and help them up if they have fallen over instead of just walking by (I'm looking at you startup kids in south park and SOMA). And you know what, screw it, I even give them a five or a ten if I have some cash on me. It sucks being poor and it has to really suck being homeless, possibly drug addicted and maybe mentally ill and I may not be helping in the long run but you know what? They just care about the food or the a couple bucks for whatever and I care about not feeling like a tool walking by a destitute person and doing nothing for them when I know I can make their day a little better with money I'm just going to spend on overpriced coffee.