This is some information I got from Ragnars Urban Survival, an e-book I downloaded from somewhere and it is a great philosophy to have.
Everything you do when planing to sustain yourself for an extended amount of time should have 3 seperate methods of supply. None of them should be dependent on the other in any way. Think about how you can apply this to each situation
Water - 3 independent water sources, how renewable are they?
Shelter - what if one is comprimised or you lost a way of getting there?
Energy - fuel runs out... what else are you going to use?
Food - how long will your supplies last?
Safety - what if your firearm breaks?
Go ahead and post some of your ideas and suggestions today on how to use this rule of three, I would like to see how everyone is already using this, I will post mine here in few days!