Inga has put up a video about how to prepare plastic bags for knitting and crochet, as well as cutting up clothes.
I'm currently crocheting a plastic floral display for an exhibition on recycling and re-using.
I've found that the cheaper quality supermarket bags are much easier to work with, they are very much like paper, while the high street branch bags (the shiny ones) tend to stick really badly to the hook and knitting with those is a nightmare. But I've found a solution: sprayed WD40 onto a cloth and ran the plastic strip through it. Works like a miracle, the stitches fly off the needles. But it has disadvantages: it stinks and dries quite quickly, so I had to re-spray every time I left the knitting for a day or two. I still prefer to crochet with plastic bags, at least there's only 1 stitch on the hook at a time.
