I have a confession. I love information about surviving in the bush. And of course, me being me, I always have to try out techniques and methods whenever I can just for the practise and in doing so figuring out what actually works in works for the wet, cold, and slightly scared hiker. So, you find yourself in a survival situation. Let's restrict it to hiking, which normally means lost or injured. Yes, this could be extrapolated to plane crashes and so on. For that sort of thing check out Lofty Wiseman's SAS Survival Handbook SAS Survival handbook, and the US Army FM 21-76 Survival Manual.

So, you find yourself in a less-than-optimal situation. What do you do? The old British Army mantra was "If you're lost, sit down and have a cup of tea!". This is not as stupid as it sounds. We can be a bit more formal in this thinking by, as one does, looking back at the Korean War. Oops, I mean 'Police Action'. An American, Colonel John Boyd, was tasked to answer an important question - Why were the Americans getting their asses kicked in fighter jet dogfights (F86 Sabres vs MIG 15's) against their Chinese counterparts? In the short form, it was to do with agility of thought and hardware. Indeed Boyd was influential in the development of the F15/F16 series - which had more extensive canopies so the pilots could see the enemy first, which in combination with lighter and more maneuverable aircraft could kick the asses of the others. For you military historians, yes I know the Koreans were cheating by using battle-hardened Soviet pilots. Don't get picky. As a further aside, Boyd was one of the strategists for the first Iraq war. Interesting guy.
I digress.

This was formalised as the OODA loop - OBSERVE the situation, ORIENT to the problem, DECIDE on a course of action, then ACT. In other words, sit down and have a cup of tea, work out what's happening, decide what to do - then do it.
Using this reasoning, first you need to recognise your are in a survival situation in which your well-being is no longer assured. Second - what is more likely to hurt you worst and first? There are a few acronyms about the order to develop a survival plan. I take a more fatalistic view - what's going to kill you first? Hey, it's a Zen thing. And the joint winners are... Water and shelter. Although hypothermia can kill you overnight and dehydration can take a few days, I consider water above shelter because if you have your pack, you already have shelter materials. There's some wiggle room there. This order of course changes if you've just plunged into an icy lake with howling cold winds around you, and your pack has fallen off a cliff. Food, needless to say, is way down the list of priorities!
Lets start with my personal favorite - Water.