If you buy tinned tobacco and jar it, or buy fresh bulk tobacco and jar it, it will stay moist enough. With tobacco that has gotten somewhat dry to the touch, you can rehydrate it easily enough -- various methods described in pipesmagazine and on on Forums posts, etc. Simple. Tobacco can get pretty dry and still be smokeable. Five Brothers, for example, comes dry and smokes dry, as does Tabac-Manil Semois. Only when tobacco gets so dry it is fragmented into dust at the bottom of the jar or bag is it stale; it's durable stuff. If you introduce too much water, you'll make mold, so use approved methods.