Sometimes the things you least expect can come in handy. It may sound silly, but if you're lost, a bottle cap might be your saving grace. By learning how to create a whistle from an old bottle cap, you'll make yourself more audible over more distance, and increase your chance of being heard and found.
Take a bottle cap from a water bottle, pop bottle, or beer bottle and dry it off.
Hold the bottle cap so that the deep end (inside) is facing up. Place the middle of your thumbs over the middle of the cap so that your knuckles are touching each other, and create a "V" shape.
Put your lips right up to the spot where your thumb knuckles meet, and blow as hard as you can. If it doesn't work at first, try adjusting the angle of the cap or you can try bringing the tips of your thumbs closer together.
Experiment with different caps to create different sounds. You may find that you prefer one over another.