I haven't had hard brown sugar in years. Perhaps that's because I keep several pieces of clay flowerpot shards in the container.
A number of years ago I broke up an already hopelessly cracked/broken clay flowerpot. I chunked it into pieces about the size of a vanilla wafer. Ran the whole lot through the dishwasher several times, then put a couple of chunks in my brown sugar. Never had a lump since then.
P.S. It's a whole lot cheaper than the little "brown sugar bears" they sell in catalogues or in cooking stores.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!