Sweat

The human body is capable of keeping itself cool in extremely hot conditions. Our body's natural air conditioning? Sweat.

The principal of sweating is fairly straightforward. Your body secretes a liquid onto the surface of the skin. This liquid evaporates, taking heat from your body as it does so. The blood passes through this newly cooled skin and recirculates to the rest of the body, cooling it as it does so.

Sweat is necessary for survival in hot environments, but can turn deadly in the cold. Risk for hypothermia increases exponentially when you are wet. Much of survival is centered on keeping you dry, because if you are dry, you can keep yourself warm. You need to be extra careful of keeping yourself dry of your body's own coolant. If you are doing intense work in a very cold environment, building a snow cave for instance, or trudging through waist deep snow in snowshoes, every effort should be made to tailor your exertion to keep yourself from breaking sweat. Take frequent breaks and strip some layers if you begin to feel hot or warm.

Cold kills in the wilderness. And if you're wet, you're cold.

Water

Water is absolutely fundamental to life. The human body can survive for upwards of a month without food, but will shutdown in less than 3 days without a steady supply of our planet's most abundant liquid. To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors. In a survival situation, 4 liters (approximately a gallon) should be your daily goal.
Thirst is not a proper indicator of when you should drink water. Thirst is a sign of dehydration, dehydration that you are trying to avoid. Instead focus on the color and frequency of urination. If your pee is yellow, you should be drinking more water. If your pee is clear or very faintly colored, you can rest assured your water consumption is adequate.