If your brown sugar has transformed into massive chunks that are harder than granite, a quick bake in the oven can turn them back into piles of soft, usable grains. Photo by david pacey.
If you're dealing with some really stubborn chunks of brown sugar, Teresa Floyd at Food52 recommends you throw them in the oven. Just heat it to 120C, spread the chunks out on a parchment-lined baking sheet, and let it heat for about five minutes. Be sure to watch it, though, otherwise it might begin to melt. If the chunks won't break apart after five minutes, heat in one-minute intervals until it does. You can also de-clump brown sugar in the microwave, but you may not have one around, and this method is better for dealing with the really hard stuff. Once you have it all soft again, you can keep it that way with a slice of bread.