Some things I collect I like to keep in jars - I think a jar suggests that something is being preserved - that the something in the jar is precious or interesting, or just worth keeping. They signal something a little different to a 'specimin bag' type thing which I have also used. I collected scraps in jars whilst walking, these jars hung on the wall are part of a piece I showed for my recent second year assessment.
I've been trying to keep this practice up over the summer so that I have plenty of material to work with when I get back in the studio in september!
The things I've been collecting have been a bit more varied though - not just human 'left-overs'. I collected feathers and found broken bird eggs on one walk. It's that word again 'TRACE' - I can't get it out of my head, I think it's going to be really important in my work now.