My mother and grandmother always keep a slice of bread in their canister of brown sugar, and when I started baking, I realized why. It will keep your brown sugar soft and stop it from getting rock hard and clumping up in your canister. So, just stick the heel of a loaf of bread in there (I replace it occasionally with another heel of a loaf).
To be doubly sure that my brown sugar stays soft, I also have one of those special brown sugar terra cotta discs in my brown sugar canister. I dampen it occasionally, and set it in an old 1/4-cup measure (so it isn't resting directly on the sugar). You can find them on-line @ eBay, Etsy, or any store that has baking supplies.
BUT ~ what do you do when you need your brown sugar NOW and haven't done either of the above precautions ~ and your brown sugar is rock-hard? You can soften it in the microwave!
1. Put the amount of brown sugar in a microwavable bowl.
2. Put a couple of damp paper towels on top of the brown sugar.
3. Tightly cover the bowl with plastic wrap or microwave safe lid.
4. Heat the bowl in the microwave for 30 seconds. (If not yet softened, continue to heat for 10- second intervals.)
5. Fluff the sugar with a fork or whisk and use it immediately
6. Put that piece of bread in your canister to soften what's in there for next time!!