Over time the moisture in brown sugar evaporates and the sugar hardens. To remedy this problem, you have to find a way to put the moisture back, and that's just what these tricks do. Soften it back up with one of these simple tricks.
To resurrect your hard brown sugar, grab a slice of bread. Any old piece will do, but the crustier the bread, the better. Pour the sugar into a canister, and place the slice (cut-side up) on top of the sugar then cover it with a lid. Keep overnight, minimum—and the moisture in the bread will have turned the granules soft and separate. The bread will have absorbed some of the molasses, turning the top layer of sugar lighter brown or white, but you may just discard that top layer and proceed with the rest. (Keeping the bread cut-side up will minimize the discoloration.)
Alternately, you could place a few apple slices or a slice of bread in an air-tight container with the brown sugar. Then, remove the slices once the sugar has softened, place the brown sugar in a bowl, and cover it with a damp cloth. Let it sit over night. Next day, the sugar is fresh as new.
You could use the paper towel trick if you need to re-hydrate the sugar the day before. Place a piece of wax paper on top of the sugar, then a damp paper towel on top, and seal it. It'll be good as new the next day. Don't leave the paper towel in there for too long though, otherwise it may grow mould.
If you need it immediately, place the brown sugar in a microwave-safe bowl, and cover it with a damp paper towel. Microwave in 20-second increments, until the sugar is soft. Use your fingers or a fork to soften any clumps that remain.
Warning: The sugar will be hot, so handle it with care.
The Best Way to Store Brown Sugar
Two words: Air-tight. Air is the enemy of moist brown sugar, so you’ll want to store it in a container that’s completely cut off from air flow. Roll up the bag tightly, squeezing out any air, and wrap it securely with a rubber band. For extra precaution, store the original bag in a sealed plastic ziplock bag.