Be enthusiastic -- to get rid of plastic.
Use your canvas bags intones
Tim Minchin on his Environmental Anthem -- filmed for a BBC comedy show.
I recommend that you be patient...the video moves from the solo droning man to a musically joyous group happening.
Edited by Rebecca to add:
My mom and I both carry a reusable bag in our shoulder bag/purse. There are plenty of places to buy them, but here's a great list of patterns to sew, knit, or crochet your own reusable bag.