A slice of apple works well too. Or you can buy a little clay disk, soak it in water for a while and toss it in your sugar container. It keeps it forever. I keep my brown sugar in Tupperware and it never gets hard.
I have a terrible time keeping brown sugar soft...it is so dry where I live. I've used the slice of bread in the bag trick - it works well. Out of desperation one day, I decided to vacuum pack my brown sugar. It worked great! I just make the food saver bag extra long so that I can cut it open then vacuum pack it again and again.