I have in the past, when I have sold art, placed thank you notes and leaflets/stickers on animal cruelty with the works.
Pretty much any time I have to send something out... christmas cards, birthday cards, I will place stickers or something in there
It's hard knowing whether your leaflets/stickers are going to go to waste or good use. I figure that with stickers, you should try giving them to children. They LOVE stickers! I have found that a parent will be curious about the sticker on their child's shirt and then go 'hmmm' about it, lol! I'd like to think that they are thinking about the message and not 'how did that get there?' or 'where did my child get this?'
Leaflets, which have more graphic information/pictures, are best suited for younger people, ie uni students. They are so much more open-minded than say a highschool student whose parents are still mollycoddling them, lol!

And uni students are more willing to experiment with new things like vegetarianism...
Hope that helps...?
EDIT: BTW, You can try handing leaflets to older people... however, they are usually so set in their ways that they don't want change, nor do they like being told to change...