I reached my limit months ago: I was sick of seeing people litter, specifically on the Red Line el train. This is what I did:
I saw a woman unwrap a package of gum and let the paper fall to the el platform. It particularly bothered me because she had a child with her and this is a terrible thing to model for your children. I walked over to her, picked up the wrapper, stood before her, looked her in the eye with a generous smile and said in my warmest voice, "Please don't litter." She muttered something as I turned away and threw her garbage in the trash can. I then resumed my post, waiting for a northbound train.
Soon after that I saw a couple of teenagers sitting on an el platform bench. She sat in his lap and they seemed very happy and giggly. They had ice cream bars and the girl let her wrapper fall to the ground. I walked over and knelt right in front of them, which clearly startled them because they went silent and stared at me. I picked up the wrapper, stood before them with it and said, "Please don't litter." Again, I did it in the nicest tone of voice, as if I were asking them for a personal favor because littering personally hurt my feelings. Again, I didn't wait for an answer as I turned away. I dropped the ice cream wrapper into the garbage as I heard their giggling resume.
I've now done this several times and it has become a personal mission. I HATE LITTERING. It's rude and lazy and rude and lazy. Whenever I see anyone litter, I now walk over to them, pick it up and hold it in front of me as I say, very nicely and usually with a smile, "Please don't litter." I am beseeching them, appealing to the wonderfully good person I know is inside them, or at least that's what I want it to look like. It's really the most passive-aggressive behavior I've ever indulged in. I'm seething inside and sick of the garbage these idiots leave, but my current strategy is to "kill them with kindness." Join me.