Here are few things that I like to keep in mind when putting together a survival kit.
What's the plan?
This is the biggest and one of the most difficult question - what will you need this kit to help you accomplish? An escape and evade kit is going to be different from a wilderness survival kit, which in turn is going to be different from a kit for surviving in urban sprawl or a get home bag.
Your survival plan should directly guide what you pack.
Weight & Size
What are the constraints on the kit? How big can it be and how much can it weigh? Is this an altoids size kit, a bug out bag or something you're going to throw in the back of an SUV?
Weight is huge if you're going to be carrying it on foot--for most, a 70 to 80 pound pack is a no-go. Don't feel like a wimp for having a pack with a manageable weight - in the original Delta Force trials, they gave the candidates 40 to 45 pound packs for long distance hikes, and that was hard enough for them. You want a pack that you can carry all day, over rough terrain and moving at a reasonable speed.
If your pack is going to ride in the back of your vehicle, weight becomes less important. If you really can't cut the weight, organize the pack into sub loads, where you can ditch the heavier/less critical items if necessary.
Ragnar's Rule of Threes
Things don't always go according to plan. If something is essential to your survival plan, have three ways of getting it done. For instance, a long term wilderness survival kit would want three methods of starting fire, acquiring food, boiling water and so on. This is task-specific, not gear specific--you don't need three knives, three axes, three rifles and so on, but you would want three different cutting tools and three methods of defending yourself or hunting.
Keep this rule in mind when developing your plan and the kit to support it. For smaller kits, knowledge of primitive methods and improvised techniques can help make up for gear that you're unable to include.
It can be easy to go overboard in any particular area--I most often see this with knives. I love knives as much as the next guy, but you don't need six different knives of the same basic kind. It's wasted weight in your pack. When trying to meet Ragnar's Rule of Threes, you want versatility--a small axe, a 4 to 5 inch fixed blade and a multi-tool, for example.
It's just as easy to go crazy on consumables like ammo, batteries and food. You'll have to make your decisions here, but you will have to make compromises based on your constraints. If you have a vehicle or cache sites, you'll have much more leeway here. A pack? Less so.
The Pathfinder 10 Piece Kit
Dave Canterbury's 10 Cs of survivability are a great general guide for survival basics. Your individual plan may not require some of the longer term or wilderness focused items, but a good list to review none-the-less.
Some Commonly Overlooked Areas
These are often overlooked in survival kits of all shapes n' sizes. Your mileage may vary depending on what you want to do:
- Trauma Kit: Not just some bandaids, but a kit capable of responding to a serious, gunshot wound level injury. Stop the bleeding and keep the patient alive until help can be found.
- Currency: Cash talks and gets things done. If you have zero faith in dollars, gold is the way to go in terms of portability.
- Spare Parts: While we like to think that our critical gear will never fail, that's not always the case. Spare parts--in the form of springs, firing pins and so on--weigh little, take up little space and are very difficult to impossible to improvise on the go.