For each cup of sugar, add 1 to 2 tablespoons of molasses for light brown sugar, and up to 1/4 cup for dark brown sugar. Depending on the recipe, you can either combine them in the food processor or just mix them together with other ingredients.
We like doing this for a couple of reasons. First, as we already mentioned, it saves us from having to deal with hardened clumps of brown sugar. (Although, there are tricks for dealing with that problem, and products to help prevent it.)
This method also allows us to control the amount of molasses flavor in the dish. We don't have to buy light and dark brown sugar – we can simply adjust the amount of molasses to fit the recipe.
Do you make your own brown sugar?