Interesting reading people in the U.S. generating ideas on this thread. When I lived in the States, I wouldn't have done a very good job planning. Now that I've lived in post-conflict and conflict zones for a few years, I have a different perspective. I'd say the best preparation is having lived in a hot zone or an area that is immediately post-conflict. Even a month spent somewhere without running water, where the grid is only up 12-18 hours a week, etc. will train you better than any hypothetical situation you can dream up. Living without electricity (and hence without refrigeration) is the key challenge here. As people mention, making water potable is not difficult. But the changes you'll need to make to live in an area like this (I live on the Uganda - South Sudan border) are not radical, they're subtle.
Don't kill a chicken until you're going to eat it. Build a schedule that your friends and allies can depend upon, but not one obvious to those watching. Figure out what knowledge you can trust, verify what you can. Standardize with your friends: everything 12v DC, everything .45ACP or 7.62NATO, everything valuable moved by daylight, etc.
Build rules and live by them. It matters less what the rules are and more that everyone involved understands them.
I was in la Côte dIvoire when things were bad, was in Lebanon during the bad days, etc. Where I live now is post-conflict, but being aware of your surroudings is key. There is a UNHCR/IDP camp (internally displaced people camp, basically a refugee camp) of over 20,000 people walking distance from where I live. You aren't going to have enough ammunition if things go wrong, but you can cover enough ground to get away if you know what's up in time.
Know your surroundings, know who your friends are, know the language, know the frequencies, know how to use your tools (offensive and defensive). Never go to bed without your gear in order, never wake up without knowing where you are.
That's all I've learned in this long process. I've made a lot of mistakes and been lucky none of them has killed me.
Just keep learning.