I use cardboard in my garden and my compost piles. For the garden, simply lay it between the rows and cover it with grass clippings, leaves etc. This will keep the weeds down and help preserve soil moisture. I'll take my lawnmower and after tearing up the cardboard into smaller pieces, I'll run over it with the mower to shred it before putting it in the piles.

For you folks that are ilde due to winter weather and want to start preparing your spring plot, take the cardboard thing a bit farther and lay it over the entire area and layer it with leaves, grass, manure and let it compost over the winter. By springtime the stuff will be ready to turn into your soil for some much appreciated organic matter. Plus, most weeds should be long dead by then.