I was standing at the bus stop this evening with a group of teenage girls. they were eating that crap masquerading as food, that comes from McDonalds. One of them threw her McD's bag on the ground. I gave her a hard stare, and realised she was looking sheepish. In fairness, I did look round and could not see a litter bin. Nevertheless.
Slowly, deliberately, I leaned over, picked up the bag, and acting tentatively, and very sweetly, with a slight questioning tone in my voice, even though I felt assertive, I said, I think one of you dropped this." The girl took it back, and maybe later deposited it on the bus. But I think, hope, they will all think twice in future. They looked the type who would understand and think.