Everyone has "tricks" to keep their brown sugar soft. I have heard them all, and even tried them all. (Anyone ever put a piece of bread in your brown sugar container?) And guess what? They NEVER work! Sorry, but they don't.
A million years ago (maybe like 10) when we got our vacuum sealer, I started putting my brown sugar in jars and then sucking the air out - it worked great! Then, I got lazy and stopped using the sealer. It STILL worked great!! I have been putting my brown sugar in jars ever since, and never ever ever had one speck of hardened brown sugar. Seriously. It's that easy. Who knew?
When I open a new bag of brown sugar, I just pour as much as I can into a wide-mouth jar* and then poke it down with the tip of a spoon. I don't pack it tight, but kind of jab at it, so it fits better and gets any lumps out. I keep doing that until it's almost to the very top. Then I just store them in my pantry to use as needed.
*I use wide-mouth so it's easier to spoon out. And I also like the smaller jars, but you really can use any kind you want.
So that's it! I recently converted my sister to this jar method and she loves it! Try it and see what you think.