The Internet offers all sorts of ways to soften up rock-hard brown sugar: microwave it for a few minutes; throw in a slice of bread or a water-soaked terra cotta disk. These ideas are grand, but this is my preferred method for keeping brown sugar soft (and for "rehydrating" it): throw a marshmallow into the bag or air-tight container! I think you'll agree it's the cutest of the ideas out there. Just be certain to let it sit over night so it can effect its softening magic.
Source: popsugar.com