In a true survival situation, such as if you’re lost in the woods, the first priority is to do everything you can to maintain your core body temperature. Hypothermia (core temperature too low) can kill a whole lot faster than a lack of food or water. Hyperthermia (core temperature too high) is no fun, either.
The first step is to get out of the elements if at all possible. Rain, snow, wind, and even the hot sun can all negatively affect you. Avoid sitting on damp ground or cold rocks. If you have a jacket or something with you, use it as a cushion to help avoid losing body heat through conduction. Insulating yourself from other objects is important when trying to maintain your core temperature.
Our first line of defense is our clothing, of course. Always strive to either wear or have with you seasonal appropriate outerwear. Even a small rain poncho stuffed into a pocket or pack will benefit you should the weather take a turn.
A good quality emergency blanket will also work well. I stress, though, that you should purchase one of good quality. The cheap ones, such as you might find at a dollar store, are so thin and fragile they are all but worthless. Spend a couple of extra dollars and get something durable, possibly even wool. More than one hiker has unfolded their cheap emergency blanket and found nothing but ribbons of material because it had worn through on all of the folds in the package.
Emergency blankets work best when wrapped tightly around you, like a cocoon. However, they can also serve as a roof for an expedient shelter, keeping the rain and snow off of you. Most of them don’t come with easy attachment points where you can tie paracord (you do carry paracord, right?) but you can make your own grommets, after a fashion. Take a small rock and place it in the corner of the blanket. Fold the blanket around the rock a couple of times, then tie your paracord around the resulting bulge.
A small campfire can also serve to warm you up and dry you out. This is why every survival kit, no matter how small, should have fire making gear in it. A butane lighter, strike anywhere matches, and/or a ferro rod, coupled with tinder like dryer lint or cotton balls soaked in petroleum jelly, will make this job infinitely easier than trying to assemble a bow drill or some other primitive fire making apparatus.
If the weather has been rainy and you have a hard time finding dry wood, try batoning firewood. Yet another way is to use a pencil sharpener to carve off wood shavings from thin sticks. That should provide enough small fuel to at least get the fire started.
But, what if the problem is too much heat, rather than too little? Baking in the sun will dehydrate you quickly, adding to your dilemma. Use your poncho, emergency blanket, or even a shirt or jacket to create shade to rest under. Limit your activity as much as you can.
If you have a body of water nearby, such as a pond or stream, soak fabric and place it on your neck and wrists. I do not recommend you use your available drinking water for this, though. Consume your potable water to keep hydrated.
Maintaining your core body temperature is absolutely crucial to survival. Be sure to have the proper gear with you any time you venture into the field.