If you land in a worst case survival scenario you need to do two things -- stay alive and find rescue. If you're cast away like Tom Hanks and you can't signal for rescue, then you may as well get used to talking to that volleyball. While smoke signals are a legitimate form of emergency signaling (three quick puffs) people aren't exactly on the lookout for them. A signal mirror is an option, but if you want an unmistakable signal that no plane, helicopter or ship will miss, you need to go with a flare.
There are many different types of flares to choose from. Some require a gun and shoot into the sky. Others are handheld and emit a red flame that you hold and wave over your head. Many car emergency kits come with flares, so check your trunk if you've crashed your car or run out of gas in a desolate area. The same goes for ships and planes, so search any wreckage you come across for rescue flares. If you really want to go high-end, you can spring for a laser flare. It casts a beam that can be seen day or night up to 30 miles (48 kilometers) away. They cost about $250, but you can't put a price tag on your safety.