Unless brown sugar is stored truly airtight, it will harden over time and become impossible to work with. There are lots of little tricks for keeping the sugar moist, like adding an orange or apple peel to the sugar, or a crust of bread (that’s the one my mother taught me). But lately I’ve been using a little reusable terra cotta brown sugar bear. Once soaked in water, it will keep brown sugar moist for about three months. Check your local kitchen-supply shop for brown sugar bears (also known as brown sugar softeners). On-line they can be ordered from Sugar Bears, Inc.