I had a chat with a highly experienced pilot friend of mine. This guy has well over 4000 hours, mostly bush flying in Alaska, winter and summer. He said neither he, nor any other pilot he knows, is really all that familiar with the ground signals. He said he's seen the card somewhere or another, but doesn't really remember them, nor does he keep a card in his plane.

He pointed out that unless a pilot is flying in an organized search, he really isn't looking at the ground much, rather he is looking around for other aircraft. He also pointed out that a mirror flash, unless it is flashing in some recognizable pattern, might not be so good. A single flash is too easy for the pilot to think it is just the sun reflecting off of a puddle or something. And in Alaska, one can't count on having sunny days.

He also said that smoke and fire, the bigger the better, is probably the best way to get attention. He said that if you are going to spell something out on the ground, plain old "SOS" is probably best. It might be best to combine these. A smoke plume to catch the pilot's eye, and a big "SOS" to indicate you need help.