When I was in the military overseas I was fortunate to go on a few bush-survival courses and of the things which have stuck with me over the years, a big one is what they called the Rule of 3s.
In short, it's a mnemonic aid to help you prioritize your needs and basically it means that, in extreme conditions, you can survive for:
* 3 minutes without Air.
* 3 hours without Shelter.
* 3 days without Water.
* 3 weeks without Food.
Some folks add "3 months without Hope" to that list, which is grim but probably quite true.
Not much point in trying to find water if you're on the verge of freezing to death, you know? It's a pretty easy sequence to remember - so if you're ever stuck, arrange your priorities accordingly.