Survival has different meanings to different people and ranges from becoming lost in a large metropolitan city to an end of civilization event where the continuance of humanity is in question. In our case we will consider wilderness survival where we have become separated from our group and we need to make do for a short period. For both extremes, the same rules apply although the preparations will differ.
The greater your knowledge and skills, the fewer tools you need. The first step should be to stop and think. For what am I preparing? How long am I likely to be on my own? How much space do I have and how much weight can I carry? How much am I willing to spend? Next, you must determine the size and weight of your kit(s). If you are assembling something to keep in your pocket or briefcase, you will probably have few tools and those that you have will be of a miniature variety (i.e., a wire saw). Finally, you must think about how much you are willing to spend. On the other hand, quality matters, so don't be mislead by cheap low-quality gear. Try to make sure each item has more than one use. And KISS!
Personal survival kit contents
Here's an interesting article by Pete Kapustynski
Collected Wisdom - Survival Lists ( v3.0 Last Updated: August 1996) - This is a compilation of survival kit contents ranging from the simple to an end-of world-as-we know-it. Makes interesting reading as to what people consider to be essential.