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. Auto hubcaps containing rags soaked with crankcase oil are a good improvised signal.
A military VS-17 signal panel takes up little space in your kit, is highly visible and is 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, 20 feet 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 attract attention. Pyrotechnic signal flares are bulky, a fire hazard and are a one-chance deal. Much better are laser signal devices designed for this purpose, such as the Greatland Rescue Laser. http://www.bestglide.com/rescue_laser_flare.html
Everyone outdoors should carry at least a simple whistle. It can be heard for a greater distance than yelling and it takes much less effort.