Microwave the brown sugar with a damp paper towel for 10–20 seconds
Microwave it with a slice of bread for 30–60 seconds
Microwave it with a marshmallow for 20–30 seconds
Place it in an airtight container with a piece of apple overnight
Place it in an airtight container with a slice of bread overnight
Break up clumps with a fork after softening
Store it in an airtight container to keep it soft