It depends a little bit on where you want to signal emergency. But I think you're talking about remote areas. In these areas, searches are most often made by aircrafts. That's why it is necessary that you're seen from above and from far away. So you have a couple of possibilities dependening on your equipement. There is also an Wikipedia article that ...
WHAT ARE YOUR OPTIONS?
You have three main choices:
A dedicated PLB (Personal Locator Beacon)
A satellite messenger with added rescue facilities (DeLorme or Spot)
Conventional measures such as a whistle, strobe light, mirror or flare.
For any non-trivial situation, I believe the PLB is unambiguously your best option if rescue is your top priority.
In general, anything out of the ordinary can get the attention of a search party or whomever.
During the day, a smoky fire is more visible than a bright one. Build the fire, then add wet material to make smoke.
Be sure to read the Wikipedia article on distress signals.
Before going in to the wilderness, make a plan for how you will deal with an emergency ...
You can build a so called "smoke generator" and feed it with certain material to specifically generate white smoke. It's basically a survival technique.
First, make a platform from dead wood, and stack it as shown in the illustration.
Stack the wood 4 or 5 high, and make it fairly large, perhaps 4 feet
in length. Then cover it with evergreen boughs, ...
It sounds obvious, but check that you haven't got anything like a sighting compass on you or anything else with a mirror - I once heard of someone who was rescued but the process was more difficult because of his lack of a mirror. Later investigation found he was carrying such a sighting compass the whole time!
If you really haven't, then find the shiniest ...
It is generally a good idea to carry some sort of emergency signaling equipment if you go into the wilderness.
Whistle : has limited range but is especially useful if you get separated from your party in fog or snow or if it is likely the peopel will be searching for you on foot. Internationally recognised distress signals are SOS in morse code (... --- ......
I think someone may have mentioned a knife being an emergency kit essential item. Well, here is one great reason: A knife can be used as an improvised signal mirror.
From today's news:
Three Lost in Oregon Use Knife as Signal.
I try never to go out into the woods without a good knife. I now have one more reason to never leave it back at home.
See the question and answers in What order of response time can I expect if I hit the "911" button on my SPOT Satellite Personal Tracker?
We've scorned such wimpy behavior (carrying a satellite personal tracker) for years, until a good friend gave us one, and insisted that we carry it. We've never had to use the emergency feature, but after each ...