A bit late on this topic, but here's how I do mine. I have not had a problem with the brown sugar. My rub has 1 part brown sugar, 8 parts other stuff including white sugar.
For containers, I re-use the big spice bottles from Sam's/Costco that the garlic powder, onion powder, ground cumin, pepper, etc come in. They are a great size for me and have shaker caps so the rub is easy to apply. Then, after I mix up and fill the containers, I just stick them in the freezer. I do make sure I have the rub well mixed, so maybe the powders kind of combine with the brown sugar granules and it doesn't stick back together, and I fill the bottles loosely, I don't compact them at all. At any rate, my mixes have stayed 'shaker loose' for 4-5 months. Don't think I've ever had any in longer then that before they get used up.
Another way to keep brown sugar soft is to put a slice of apple in the container. Keep it so the skin sits on the brown sugar. The moisture gets drawn out of the apple. Replace it every couple of months. I use about a 4th of a small apple. You can refresh a container of hard brown sugar this way, just takes a couple of days.
Now, this thread has me wondering if putting a couple of hunks in a rub bottle might make for a more interesting rub flavor...