How long-term are we talking? Spices lose flavor over time no matter what you do, so anything over 6 months you might as well be using sawdust, in my opinion...
I'd guess that a vacuseal container or bag, in the freezer, would be the best bet to make it last long term.
For short term, you'd want an airtight, lightproof container. Some kitchen stores carry airtight metal tins intended for spices, perhaps you can find one large enough to house your rub?