I Tablespoon molasses (regular or light is fine) added to granulated sugar to make a total of 1 cup is a great money saver when baking cookies, cakes, etc. If more or less is needed, adjust accordingly. That is what brown sugar is made of that you purchase in the store – check the ingredients on the bag or box. This way you do not need to worry about keeping it soft too.
If you brown sugar does get hard, moisten the inside of the canister lid and replace it on the canister and it will re-moisten the brown sugar. I prefer this method. Some people use a slice of white bread or a quarter of apple or a special clay bear that you moisten and add to the canister.