Lots of great info in this thread. Just with any type of prep, the most important part is starting. Then you can slowly build it up. Kinda like getting the tools you need for your garage. If you bought all your tools at once, it'd wipe you out for sure. Start small and work up from there.
I know food rotation has been mentioned, but I find it's not that hard to do. I started by buying an extra can or two of non-perishable (or longer shelf-life). Like Katchup. I'll get 4 or 5 bottles when I only need 1. Then I put the extras in my basement. When the ketchup runs out in the fridge, I grab one from the basement and add one bottle of ketchup to the shopping list. We've got a pretty good amount of food storage now, and by slowly building it up, it doesn't cost a fortune all at once. Even though we haven't had a disaster, I can attest to how much easier life is when you think you're out of chicken and you remember you have 20 cans of canned chicken downstairs.
Kinda like finding something in your scrap metal pile that was just what you needed, instead of having to plan to go to metalmart to get more metal.