2 Answers

1. Air-Tight Container - A container with a tight-fitting lid or even a resealable plastic bag can really help keep brown sugar from losing so much moisture that it turns into a rock.

2. Terra Cotta Brown Sugar Savers - You can find these little terra cotta disks at almost any baking supply store. Just soak it in water and seal it in with the brown sugar to help soften it up.

3. Marshmallows - We found this tip through Real Simple and are curious to try it. Apparently, throwing a few marshmallows into the bag helps keep the brown sugar soft!

4. Slice of Bread - This was our mother's preferred method. She swears by leaving a slice of bread in the bag overnight.

5. Slices of Apple - This is the same idea as both the marshmallows and the bread. The brown sugar will draw the moisture out of the apples and soften up.

http://www.thekitchn.com/pantry-problem-solving-how-to-118215

My mom would just put a slice  in there with it ; or bread in  cookies to keep them from hardening