I snapped a couple of pictures that demonstrate the downside to using the typical kind of brown sugar that is still fairly moist when it's straight out of the bag.
|Unsifted and clumpy|
|Sifted and thoroughly mixed|
The second photo was take after running the rub mixture through a flour sifter.
You can virtually eliminate this issue and avoid using the flour sifter by using Turbinado sugar aka Sugar in the Raw instead of regular brown sugar. If you don't have any Turbinado sugar in the cupboard and don't want to make a trip to the grocery store, you can spread regular brown sugar onto a cookie sheet and let it sit out for a couple of hours. It will dry fairly quickly and mix more easily with other ingredients.