A large, comfortable backpack
It may seem obvious, but you really need a big backpack that is comfortable to wear. Chances are, it will end up being pretty heavy, and you'll need to keep it with you at all times. You'll want something that feels okay on your shoulders and that doesn't bounce around too much when you run. Next time you're at a big box store, try on a few different models and see what works best for you, because you and your backpack will become bestest buddies when the zombies come. It's especially important that it is big enough to hold all of your gear, and that you get in a lot of practice stacking everything inside neatly. Being efficient with your looting will make a huge difference when a second can be the difference between life and death.