What is the will to live? Basically it’s you telling Death, your body, the world, or whatever adversary is coming after you, natural or otherwise, that you aren’t going to die. You will live no matter what. Studies have taken place to show that people can extend their lives by a few days to a few weeks if they have an important event or milestone to reach (see Shimizu & Pelham’s “Postponing a Date With the Grim Reaper: Ceremonial Events and Mortality”). Also, look to stories where children or severely handicapped people have survived days in the woods without help. What drove them on to keep alive? The will to live.
The power of the mind is a mysterious thing. The mind can heal (placebo effect), and it can keep us going for that one extra step. The question is, what motivates a person to keep on living? For some people, it could be spiritual. For others, maybe it’s family. For some, maybe they just want to have that last Guinness they had in the fridge. The study referenced people surviving until they hit a birthday or other milestone. In an austere condition, such as a wilderness survival or similar situation, the milestone could be getting back to loved ones.
Now ask yourself, how can you, as the potential survivor, help keep your will to survive strong? It starts at home before you ever find yourself in a bad situation. The key to success is ensuring you do everything possible to decrease stress and anxiety. Little things, such as making sure your finances are taken care of, there's a plan in place for children or pets in your absence, or your family business (if applicable) is in order before leaving. I say leaving, because most survival situations take place outside of the home – camping, hunting, deployments, etc. If you know that all the little things are good to go, that will be one less stress factor that will eat away at your will when you’re stuck out in the woods with a broken leg, evading a pack of killer wolves, while at the same time hiding from a serial killer.
If you’re constantly thinking about problems with your life, or being depressed, it will start to dissolve your will. Focus on the good things. Keep telling yourself that you’re going to make. You can make it one more minute. You can take one more step. Think positive.
Now, you might ask yourself, how do I apply this theory in my writing? Easy enough. If you have a character in a horrible situation, first decide if this character is going to have the will to survive, or is going to get a bad case of "give-up-itis." Maybe you want to show the reader the slow decay from a strong will into the realm of giving up on life. You want to show the external factors that are going to motivate or demotivate your character. If they left things unsettled with friends or family, have it scratch at the back of their mind constantly. On the flip-side maybe the thought of getting back home to their loved ones is that little extra juice they needed to keep putting one foot in front of the other. If they're strong willed, keep their mind focused on the good things. If their wolf-bait...have fun breaking them down piece by piece until they just want to lay down and let the darkness take them.