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 great, 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 sure to let it sit over night so it can do its softening magic. There's nothing worse than deciding to make a batch of chocolate chip cookies and discovering your brown sugar won't move, so store it this way for guaranteed softness at all times.