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. I have heard an interview with the author of Meateaters Guide to Wilderness Skills and Survival which makes me believe it would be a good book. Check out General Backcountry Safety by the Mountain Rescue Association. AirCav hosts a good Survival Manual.
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. In most cases, staying put makes it easier for rescuers to find you.
You should be well versed in basic first aid. Carrying a first aid kit isn’t as important as knowing what to do. With knowledge you can typically improvise. The with advent of modern signaling technology, the emphasis has shifted from treatment of serious issues in the field to stabilizing, with the treatment provided once the victim has been evacuated.
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.
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 a number of “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. Old fashion Zippo lighters while not the lightest option tend to be very reliable provided you refill them. My experience is that the “high end” lighters are not the most reliable. The electric spark can will fail to ignite the gas… especially when over 8k feet.
I recommend bring a few firestarters. A number of items you carry for other purposes can be turned into fire starters. Cotton balls + vaseline, alcohol based hand gels, fuel for your stove, etc. Most firestarters use a 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 when the easy to pick up tinder is wet. Note: having a knife that is larger than the Swiss Army “Classic” makes this a lot easier. A more complete write up is an equipped.com firestarters page.
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. 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. 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. Andrew Skurka’s Satellite Communication is a good summary. I would note that PLBs are the most reliable signaling method. I found signaling devices that rely on Iridium network (like Garmin inReach and Zoleo Communicator) are more reliable that GlobalStar (Spot), but even with the full coverage from Irdium, there have been times when I had to move locations / repeatedly try signally for 5-10 minutes before I got a message delivered.
Many years ago I used to carry an CW (morse code) Amateur QRP rig because of it’s low power requirements. Typically 20 meters was good during the daytime, 30+ meters is good at night. I would research what frequencies/times had particularly active nets in the area I was traveling.
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 know how to use them. Just keep in mind that space blankets are not a substitute for appropriate insulation. Exposure is the number one killer in the backcountry. Read material by Murray Hamlet about starting warm. 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 page.
The SOL brand significantly better than many of the “generic” emergency blankets. The pricy 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.
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 treatments elsewhere. 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 of a lake provided it’s not stagnate
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.
Being able to anticipate and understand likely weather and environmental conditions can help to be prepared.
You should be able to find your way. Sgt Rock has a good intro to using map and compass