Using it is the best seasoning...
We've used our dutch oven for home cooking from time to time just to help with seasoning. High-fat foods such as bacon, ground beef, etc. typically help season cast-iron.
On the flips-side, baking breads and dry foods in the dutch oven can actually remove some of the seasoning.
After every meal we cook with the dutch oven we clean it with hot water (NO SOAP!), reheat it until dry, wipe a light coat of butter over it, and then reheat it for a bit. This has seemed to work good. We store ours in its original box... though a burlap bag is recommended (just haven't gotten around to it yet).
Joel