Start with the basics: food, water, energy [lights, heat, cooking]. What do you eat? Stock up on it. Buy an extra can or box every time you go grocery shopping. Or two, if it's on sale. Or three if you're feeling flush. Buy some bottled water [one of the only reasons to purchase the stuff unless your tap water is crap].
Then think of how you would do the ordinary things every day without power. How would you heat water? Wash dishes? Take a shower? How would you cook?
Look around at what you already have, like camping gear and the stuff in your kitchen. Got a can opener? Got a pan or two? You're already stocked up on some stuff; time to fill in the blanks.
Don't forget medicine or first aid supplies.
Bleach to purify water.
Firemaking tools - flint, magnesium, dryer lint, matches, lighters, cotton balls saturated in vaseline (I'm an urban dweller at the moment and I am a fire goddess! I can't imagine being unable to start a fire or light a candle if we needed to).
...I ramble...I am sure there is a lot better info in the link provided ;o)
Don't bother to stock a bit of food unless you have arms to defend it from those who don't stock food. On the flip side my stocks have kept me fed many times over the years. What are you prepping for? Answer that question and you know what to do....
Sorry to rain on the parade early. When everyone has no food, those that do are fair game.
For me it comes down to availability. There's a lot of talk about comrade O bearing down on our right to bear so arms and ammo immediately shoot to the top of my list. Point being, you have to look at the future too. If there's the possibility that something may not be around for long then you need to jump on it while you have the chance.
Also, you need to stock up on info just as much as materiel. Like G.I. Joe said..."knowing is half the battle".