If you bake as often as I do, which is almost never, then you, too, know what it's like to reach into the cupboard and pull out a block of brown sugar as hard as a brick.
As Stella has explained before, softening brown sugar to restore its original texture is as easy as reintroducing the moisture it's lost in storage. Still, that requires the extra step of weighing your dehydrated rock of brown sugar and adding just the right amount of water, so it softens up without turning into a syrupy puddle. It's useful to know how to fix the problem once you have it, but it also helps to know how to avoid it in the first place.
I've come across various tricks over the years to help preserve brown sugar's texture, including adding a slice of bread or marshmallows to the container. I've also spied moisture-retaining terra-cotta doohickeys and special brown sugar storage jars online, but I'm lazy, and those things cost money and forethought. (I do not keep marshmallows in stock at home, and I'd like to think that has been at least part of my secret to a cavity-free life, which is, pathetically, one of my proudest accomplishments.)
Somehow, though, the most obvious solution eluded me until a few weeks ago, when I was eating a bowl of oatmeal at my in-laws' summer home in Michigan. They set down on the table a glass swing-top jar with a rubber gasket, casually mentioning that it contained the last of the prior year's brown sugar.
I scooped up a spoonful, and it was still as good as new. I'm almost embarrassed to admit how amazed I was by this.
So that's it. That's all you need: a really solid airtight jar or container. I like the kind above, which is designed for canning, since its rubber gasket will provide the most airtight and long-lasting seal. But other very well-sealed containers can work, too, as can a zipper-lock bag with the air pressed out.
Le Parfait Super Jar - 500ml French Glass Canning Jar
The more airtight your vessel is, the longer your brown sugar will retain its original texture—no extra ingredients, specialized gizmos, or anything else required.