I don't usually give in the subways. Sometimes, I try to be generous on the streets.
That is not enough. The thought of preaching the Gospel to them is always the first burden. Often times, I got let down due to the fact that they thought they knew Christianity and consider themselves "very" Christians already. On the other hand, I had always felt I'm in a rush to do something else.
So, given enough time, I should be able to use my money to good use, by collecting some of their own testimonies/autobiographies. And evangelism must never be lacking