I have a couple of these:


http://www.envirosax.com/products/graphic_series/

I bought them at Powell's when I visited Portland prior to moving here, and I love the silly things. They are so compact when rolled up that I've got them stashed everywhere - one in my purse, one in my pannier, one in the glove box of the car, etc, so I am never without one. It's amazing how much will fit in them and they are super strong, too. We also keep a collection of fabric bags (some are mesh) in the car for larger shopping trips, but that's usually easier to remember. I find it's the little trips to the market or hardware store where you have too much to just carry out, but not enough to warrant bringing in the whole shebang...