Exactly, if you really want to build up an emergency food supply, do what I've been convincing my Fiancee we have to do. We recently bought a house and our pantry is completely empty, she thinks we're just filling it with food, I'm preparing for when the zombies get here from Florida.
Really though, when you're at the store if you need 1 can of peas, get 3. Then, next time you use a can of peas, buy 2 more, don't just wait for the ones you've got to be used up. That way, you're always rotating the supply and adding one to it. Same goes for all other food and if you have space, a deep chest freezer is a great way to store lots of stuff.