Before it reaches the concrete stage, but when you notice signs of hardening, place an apple slice in the bag and leave it for a couple of days.
After the sugar is hard, you can soften it by placing a cup of sugar in a covered dish in the microwave and heating for 30 seconds or so. You may need to heat it more than once, but watch to see that it doesn’t start to melt. You can also heat the brown sugar on a metal pan in a regular oven at 250°F (120°C) for five minutes or so.
To keep it from hardening in the first place, store it in the freezer.