When you brown sugar hardens, don't throw it out. Instead, put the brown sugar inside an air tight container. Take a single slice of fresh bread and lay it on top of the brown sugar. Seal the container with a tight fitting lid. Let it stand overnight and you will have sugar that is as soft as the first day you bought it from the grocery store!! Discard the bread and enjoy your "fresh" brown sugar.
The sugar will stay soft up to 3 months before it will start to harden again.