Tough call. A lot of the time, I definitely feel like I'm being taken advantage of. But at the same time, so what? It's worth it for maybe that one time in 100 or whatever where I might be as sure as 100% that I'm being taken advantage of, but I'm wrong and end up unknowingly genuinely helping someone who needs it. You just never know.
But situationally, it just depends. If I have the money on me, and I feel like I'm in a safe place where I can pull out my wallet without reasonable fear of dude pulling a knife or gun and taking the entire thing, I'll often give some. But sometimes, I don't just because I'm in a hurry, or don't have a good attitude, or just whatever--sometimes, no good reason at all. And I think that's a mistake and a character flaw of mine because, whether it's convenient or not, and whether I like to admit it or not when in the moment, the fact of the matter is that I can afford to help and can afford to be taken advantage of if it turns out that the person's motives are bad. And if I can help someone who needs it, I feel that I should (i.e. am obligated) do so, regardless of the fact that I might be wrong and might be taken advantage of. I believe my faith demands that of me.
But as to those times when I do not, my approach is simply to say "no, thank you. Not right now." and move along.