Place a slice of apple in hardened brown sugar to soften it back up so you can measure it easily and use it often.

Or

In humid climates, brown sugar often gets hard as a rock. You can put that brown sugar in the microwave for just a few minutes on low so you can use it easily.

Putting your brown sugar in a container with a lid and adding a slice of bread your brown sugar will stay soft so you can use it easily.

If you use brown sugar only on special occasions you can put it in a container and put it in the freezer or in the fridge to keep it fresher for many more months than in the cupboard.