Back in the days before World War II, pre mixed boxes for baked goods were pretty much unheard of. Women either baked completely from scratch or had their own pre-mixes in jars or bins ready when they needed them. The mixes had flour, leavening, dry flavorings, and shortening in them and only needed to be mixed with water or milk, eggs if necessary, and liquid flavors like lemon or vanilla. I've become a huge fan of these mixes after stumbling across the list posted at Grandma's Pantry.
I have the brownie and cornbread mixes in my cupboard and have used each successfully several times. The brownies are tender, moist, and impressively chocolate-ly. The cornbread is moist and dense but relatively bland, which I improved by adding honey along with the eggs and milk. In the next couple weeks, I'm going to also make up the basic cake mix, pie crust, and hot chocolate mix. I suspect the later will want the addition of non dairy creamer to more closely mimic commercial hot cocoa mixes but if one doesn't mind a "thin" tasting hot cocoa, this mix looks pretty much the same as what my mom used to make up by the half gallon jar.
As for storage, the best option would be glass gallon jars. If you don't have many lying around, having ransacked your home for recyclables, I've had great results with washing out gallon milk jugs and filling them with dry goods. The jug does need to be completely dry, of course, and I found filling them easiest with the use of a paper cone to funnel the mix in. Most of the mixes mentioned here can be stored for three to six months (you use shelf stable shortening like Crisco) in a cool dark place, like the back of a cupboard.