Part Mark Verber's The Great Outdoors
Remember the 3s: You need air in 3 minutes (and to stop severe bleeding), to get warmth in 3 hours, water in 3 days, and food in 3 weeks. The Wisdom of Abo Dude touches on most of the survival principles I learned growing up. Cody also has a book called 98.6: The Art of Keeping Your Ass Alive! which covers similar material but is greatly expanded. Check out General Backcountry Safety by the Mountain Rescue Association. AirCav hosts a good Survival Manual. There are a few gems on wilderness-survival-skills.com.
Common Sense: Before you leave on a trip make sure that someone responsible knows where you are going (ideally your detailed route) and when you should be back. Make sure they know you are counting on them to call for a search and rescue if you don't contact them within a specified time period. If something goes wrong don't react without thinking. Except in the case of the 3 minute threats, you are most likely going to get a better outcome by pausing, and reflecting on all your options.
First Aid: You should be well versed in basic first aid. If someone is injured, you need to stabilize them.
Signaling: From a pure safety perspective, it is always best to travel with someone else. That way, if one person is hurt, someone can go for help. Always bring a whistle like the Fox 40. The sound carries a lot further than your voice, and it is a lot easier on your vocal cords. Remember, three short blasts is the US standard for distress, 6 long blasts is the international signal for distress. You should know how to sustain a signal fire and carry (and know how to use) a signal mirror. It also wouldn't hurt to know the standard ground signals which can be made from tarps or items on the ground. You should not expect your cell phone to work in the backcountry. Just look at cell phone tower locations near your destination (and remember that if there is a ridge between you and the tower you are SOL). You could also check out user reports which indicate cell coverage in national parks is spotty. If you are traveling alone, especially in an area which doesn't get a lot of traffic, you might want to consider bring some sort of hi-tech signaling device. There is a fairly lengthly series at BPL about Satellite based comunications which you may find useful. My personal assessment from a couple of years ago, in order of there likelihood to quickly get you assistance:
Fire: Knowing how to build and maintain a fire is one of the most important skills you can learn. Fire can provide warmth, provide a means to make water safe to drink, makes food more palatable, and can be an effective signal device. A number of items you carry for other purposes can be turned into fire starters. Cotton fabric which is frayed, alcohol based hand gels, fuel for your stove, etc. I recommend bringing at least two devices which lets you start a fire. At least one of them should work even when wet such as fire-steel, UST Sparkie, Exotac nanoSTRIKER or spark-a-lite. I also typically bring some of the "windproof / waterproof" matches which are neither, but light in most conditions and can also function as a first stage fire starter. I commonly start my fires with a cheap mini-BIC lighter. 95% of the time I use a mini-BIC . Old fashion Zippo lighters while not the lightest option tend to be very reliable. You could also bring chemical based system such as Potassium Permanganate and Glycerin such as ChemFire capsules. I also typically recommend bring some firestarters. Many firestarters use some combination of cotton or wood base which has been embedded with with a petroleum, natural oil, or wax fuel. You should also learn techniques for starting a fire without pre-make firestarters and easy to pick up tinder is wet. Note: having a knife that is larger than the Swiss Army "Classic" makes this a lot easier. My experience is that the "high end" lighters are not the most reliable. The electric spark has really issues… especially when over 7k feet and they often seem to take two "clicks" to start up. A more complete write up is an equipped.com firestarters page. There are some good notes on building a fire at firespot.
Warmth & Dry: If you have a shelter (tent, tarp, etc) set it up to provide a dry location where you can warm up. When you aren't carrying a shelter and a sleeping bag in a waterproof bag carry an emergency blanket and fire starters for warmth and read Ranger Digest's anti-frostbite-hypo kit for how to use them. Don't let the cold get you. Exposure is the number one killer in the backcountry. Check out the interview with Murray Hamlet about How to Stay Warm, Backpacker.com - Antifreeze For Your Body, and Hal Wiess' book Secrets of Warmth. You should understand wind chill and know that you lose heat 25 times faster when you are in the water as when you are in still air (e.g. don't get soaked, if you are soaked, get dry). It is possible to get hypothermia in 50F weather if you are wet and the wind is blowing a bit. See my staying warm on my winter page for more information. The HeatSheet emergency blankets seem to be the best "standard" emergency blankets. Lifesystems Blizzard Survival Blanket or Bivy are a double layered emergency system not only reflect but have a small air pocket for enhanced insulation... making them significantly warmer (but also more bulky) than traditional emergency blankets. There was a good article about avoiding and treating hypothermia which I think was quite good.
Water: Water is life. Don't get dehydrated and make sure you are drinking clean water. Always carry an adequate water supply with tools to get more water. I list possible water treatment gear elsewhere. If you are active in very hot weather check out Desert Survivor's Do/Don't. My typical water use while backpacking or hiking is 1L for every 7-8 miles when it's 30-60F, around 1L for every 5 miles when it's 60-80F, and 1L for every <=3 miles when it's more than 80F. The best place to take water is from the top 1/2 inch of a lake. Constant exposure to UV light from the sun tends to purify the top layer of a fixed body of water.
Food: Except in rare cases, you won't need to forage for food unless you are seriously off the beaten path, nor do you need to take a lot of extra food. A "normal" healthy person with average levels of body fat can go for over four weeks without food with no long-term negative consequences (assume low activity). So for most people there is not threat to starving to death if you are without food for a few days. The primary risk issue is that it's slower for the body to convert you fat tissue to fuel... so you are more likely to get fatigued. This means that your physical performance will be down, you might not be as mentally sharp which can lead to mistakes, and it will be harder to stay warm. Focus on other skills unless you are highly motivated to learn to forage.
Weather: Being able to anticipate and understand likely weather and environmental conditions can help to be prepared.
Navigation: You should be able to find your way. Sgt Rock has a good intro to using map and compass
Classes: There are a variety of classes which team survive skills. Those which are specifically about medical treatments can be found on my first aid page.
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