Mirror flashes are the best daytime signal device and can be seen by search aircraft for miles. Smoke signals are also useful if you pre-set signal fires in threes, ready to light. Empty food cans or auto hubcaps containing rags soaked with crankcase oil are a good improvised signal.
The military VS-17 ground signal panel takes up little space in your kit, is highly visible and is also multi-purpose serving also as a ground cloth or improvised poncho.
ICAO surface-to-air signals are also useful to know. Stamp these out in the snow or beach arrange large piles of branches or rocks 10 meters high or more which can be seen readily from search aircraft:
V Require assistance
X Need medical assistance
Y Yes – affirmative
N No – negative
→ I am proceeding in this direction
At night a flashing signal strobe light is best, but signal fires in groups of three will also attach attention. Pyrotechnic signal flares are bulky, a fire hazard and also a one-chance deal. Much better, though expensive, are laser signal devices designed for this purpose, such as the Greatland Rescue Laser.
http://www.bestglide.com/rescue_laser_flare.html In the USA these are routinely issued to FEMA search & rescue, Coast Guard rescue swimmmers and military para-rescue personnel.
Everyone outdoors should carry at least a simple whistle. It can be heard for a greater distance than yelling and it takes lots less effort.