Store them in a cool, dark and dry place. I refrigerate mine. Some people keep them in the freezer, but be sure the moisture content is down to 7% before freezing to avoid damage.
Tobacco seed has a long shelf life. USDA studies have shown no decline in germination rates for 5 years when stored properly. Germination rates decline at about 2% per year there after. The ars-grin seed bank rotated their tobacco seed stock every 10 years.