I either don't look or if they do catch my attention I just say sorry.
I realize that sounds cold but I'm fine with contributing to organizations that can help these folks. Frankly I believe that many are drug addicts looking for their next fix and I don't want to enable them. On a very rare occasion, I've given a few bucks and while I likely got conned, they presented such a compelling story that I gave them something.
Now, I was in a 3rd world country one time that had no social net. There were beggers and we knew about it going in. These folks were not drug addicts and were starving. We brought power bars and the like (a whole case actually) and handed them to the folks discreetly. It was sad because they gobbled them up. It was likely their only meal for maybe a few days. While there, we also gave candy to kids we saw but that's entirely different.