I invested in a pair of the Jandd grocery panniers a month or so back that have been giving good service. They hold a paper bag's worth of groceries or other stuff with ease. I don't have a picture of the bike with them mounted yet, but may have to get out and play with the camera soon...

I did approach the problem of packing a 20-lb sack of dog food home from Petsmart... mounted the grocery panniers, strapped the bag over the rear rack, and tucked the ends of the bag into the pannier on either side. A little wobbly at first, but it worked (at least out on the flats, where I live )

I really liked Mary's rig... I've got a couple of empty detergent buckets hanging around, so may have to hit the hardware store and build a set of those, too...