Here's another tip that I've been using for a while:
It really works. I just toss a marshmallow in my brown sugar and it's always soft! Best trick ever!
I had a brown sugar baby and either my sugar would get hard anyway, or it would get moldy. Ick. But the marshmallow has been the best thing ever for my brown sugar. I used to have to buy a new bag of sugar almost every time I wanted to make something... and now I use the full bag.
Best part... my daughter gets a tasty treat of a marshmallow when I put new sugar (and marshmallow) in the container (pretty soon I'll have to put 2 marshmallows in, just to make sure the girls aren't fighting over the marshmallow). :).