A couple years ago there were some long threads on this site on this topic that digressed into the best survival firearm, etc. Key of conversation was that your survival kit has to match the scenario you are going to survive in and you have to have a pre-thought out, reasoned, practiced approach to your survival tactics and skills. Season of the year, terrain, personal experience, etc., should all be considered in assembling your personal kit.
The basic "Rule of 3's" of survival is this:
You'll survive 3 hours without shelter,
3 days without water,
3 weeks without food.
To plan accordingly, remember that being warm and dry is critical to survival. Building a fire is an energy consuming task, but it is also a friend that will dry you, warm you, and perhaps most importantly, keep you in one place until your head clears (if you just crashed your favorite plane, you will need this!). Then you'll have time to think about water, catch fish, play Robinson Crusoe, etc.
On a long trip I will usually wear a safari vest to have the most essential items on my body (if the big kit sinks with the airplane, it won't be any use to you!). In the various pockets are my required stuff (AF/D, charts,...), my fun stuff (camera, ...), and the micro-survival kit: Leatherman, pinch-type flash light, space blanket, and a Altoids tin filled with matches, fish line, hooks, cable saw, signal mirror and sealed with tape.
On ALL trips there is a sleeping bag and a small gym bag in the back of the plane with a tie-down kit ("The Claw"), Wyoming Saw, Gerbers hatchet, Gerbers folding shovel, a couple bottles of water, and a wilderness first aid kit augmented with a Swiss army knike, rescue whistle, 2 space blankets, a signal mirror, Power Bars, and a couple lighters. I also carry a shaving-kit size bag with essential tools (6-way screwdriver, wrench set, pliers, vice grips, spare baterries, mini tape meaure) and a ziplock bag with airplane ditties (screw/bolt/nut assortment, safety wire, duct tape, rag, 14g automotive wire, etc.) just in case I need fix something minor out there. Total kit weighs in at around 21-22 pounds. The only time this kit leaves my airplane is when I take it with me in someone else's airplane. I sometimes throw in a firearm, but for Wyoming/Colorado survival it just is not a critical survival item. I figure if I can stay warm, fix me, and fix my plane, I stand a pretty good chance of surviving anything. This probably isn't my complete list and you should customize your list to match your area and skills.
Last but not least: don't skimp when purchasing this stuff! That cheap Campor shovel or blanket will seem OK when you're stocking a kit that you never expect to use, but the day you need it, you will want the best equipment available and your life may depend on it working perfectly and not breaking.