This pin states that you can put large marshmallows at the top of your brown sugar and it will keep it soft and fresh. I put my marshmallows in my open bag of brown sugar about 6 weeks ago and my brown sugar is just as soft and fresh as before. No hard as a rock brown sugar. This worked great!
Original pin can be found here: http://inthiscrazylife-bethany.blogspot.com/2012/01/tips-and-tricks.html
The directions are really easy just put a few large marshmallows at the top of your open brown sugar bag and then close tightly.