Some signals are good for air searches, some for ground searches, and others for both. Many things can be done to attract attention.
Three fires in a triangle, three whistle blasts, flashes of light etc. are international distress signals. Note, in the U.K. it is 6 whistle blasts, 6 flashes of light etc.
It is strange that in the U.K. the 'international' distress signal is 6 whistle blasts, 6 flashes of light etc., while elsewhere the 'international' distress signal is 3 of each. Weird. Where is the consensus in the international community?
There is confusing information out there about international distress signals. Have a look here for an example.
In the health and fitness page you'll get some more information and tips about signaling, as I talk about the importance of getting sleep but being wary of signaling.
Amateur radio transceivers can be used for emergency communications as in the photo above. Actually, that is me in the photo in the first shelter I was in when I got caught in stormy weather. Have a look at Weather Predictions to see what happened.
Know your ground to air signals as well. Print out on a card the ground to air signals, then laminate the card to protect it from the elements and place the card in your kit and or wallet. Make more than one card if desired.
I'll list some of the methods of signaling here below.
1. A fire triangle, preferably tripod fires to protect from ground moisture, equally spaced 20 to 25 meters apart. Be certain it will be in an open area or hilltop to be visual.
2. A tree torch, is when you could light a single tree that is in the middle of an open area on fire. Pine trees with their resins would work well.
3. A single large fire if you cannot have three as mentioned above in point number 1.
4. A tinsel tree is when you attach shiny objects such as pieces of an emergency blanket, chocolate bar wrappers or other, to a tree in several places, and the wind will move them causing them to glitter in the sun, lighting up the tree. If you have a lot of extra rope, which would be great if you did in an emergency, tie the rope out across a field or other open area, and tie shiny objects to it. This will help ground personnel to find you.
5. A light windmill using a cyalume (chemical stick) light at the end of a rope which you twirl to form a circle. This can be used to signal ground search and rescue, or a helicopter above. Just angle the twirling to make it visual to the rescuers.
6. Whistle. Get a good one, don't skimp. Even high quality whistles are only a few dollars, so no excuse.
7. Flashlight, headlamp or any other sufficiently powerful lighting device.
8. Signal mirror, or other mirror to flash your target to get their attention, whether it be an airplane or ground personnel. The mirror from a mirror compass like the one in the Maps, Compass and GPS page can be used as well. However, a specialized signal mirror is best but you need to know how to use it.
9. Emergency blanket, poncho or brightly covered clothing.
10. Radio, cellular phone, satellite phone, PLB etc. Coverage may not be available.
There are other methods as well. Do a research on the internet to find out more information. Have a look at the links at the end of this section to get you started.
For signaling using fire it is smoke by day, and fire by night. Use green woods and leaves to produce more smoke.
If you happen to have as part of a wreckage some tires or other black rubber products, you can burn them to produce a thick black smoke.
This is good if you have snow in the background, otherwise you'll have whitish/grey smoke in a regular fire which would be less visible.
I'll quote a paragraph from the Health and Fitness page here, because it is important information. I italicized the portions that I want to emphasize.
"On the subject of rest, sleeping in a shelter isn't the same as sleeping in your nice comfortable bed at night. There can be bugs, it can be cool, cold or hot, you may have a fire to tend during the night etc. It all depends on the shelter type and your circumstances. Get rest whenever possible, but try to not sleep during the daytime when search and rescue may be looking for you and when you can attempt to attract attention to rescue aircraft should one appear. You need to be ready to signal the aircraft and if you are asleep you may have missed the chance to get out of your predicament.
At the very least if the conditions permit, have a good smoke fire going while you rest beside it if you absolutely must rest during the day. Try to keep an eye (and ear) open every now and then, to see if an aircraft is near and to be certain that the fire keeps going. If you are in a group, take shifts resting or sleeping as appropriate."
Be certain to think before you start throwing modern items into the fire, because you may need them elsewhere, and you could find another way to make the smoke you need. Make the decision according to your circumstances, but you must consider the importance of what you have at hand.
Always leave a note and indicators of your intentions to search and rescue, if you had decided to leave the camp because you felt you needed to go elsewhere. This could be because you felt that the rescue was aborted, or your local resources are dwindling. Let them know how many are in the group, where you intend to go etc.
Have a look at the Survival Tips & Tricks page for more information.
Here are some links to help you to learn more about fire in an emergency situation.
Also have a look at the Videos page for more information.