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We all have the same issue with brown sugar. If you leave it in the container you purchased it in you end up with brown sugar that is solid as a rock when you're ready to use it. There are several ways to avoid this calamity.
Airtight Container: I store my flour and sugar in the Lock and Lock 8-cup plastic airtight storage container. Any truly airtight container will keep moisture out and will keep your brown sugar soft for a really long time.
A Terracotta Disk: If you use a container that may not be airtight you can purchase a Brown Sugar Saver. Apparently, it is just a piece of terracotta. (Some even claim you can break off a piece of a terracotta pot and use that, instead.) If it's good enough for Giada DeLaurentis, it's good enough!
A Slice of Loaf Bread: I have been told that you can also use a piece of white loaf bread in your container. The bread absorbs the moisture and will not mold, but instead get hardened and stale. I just read that this works to bring your brown sugar back to life if it's already hard. (Just leave bread in the container for 24 hours.)
An Apple Slice: You can supposedly place an apple slice in with the brown sugar, too. I assume it will get hard and shrivel up?
So if you have tried any of these and think one works better than another, please comment!