It doesn't take long for brown sugar to become hard as cement. You can either buy new brown sugar for every recipe, or learn to soften the brown sugar you have.

There are 2 main methods for softening brown sugar. One is more immediate, the other takes a day or two.

More Immediate
Place the brown sugar in a microsafe bowl. Place a wet towel over the top. Microwave in bursts of 30 seconds until soft. During breaks, use a fork to break up the clumps.

Day Or Two
Store your brown sugar with a slice of break or an apple in an airtight container. It will gradually soften over the next day or two.