Rather than posting copyrighted material...

V = Require Assistance
X = Require Medical Assistance
N = No or Negative
Y = Yes or Affirmative
-> = Proceeding in This Direction
++ = We Have Found Only Some Missing Personnel
XX = We Are Not Able to Continue, Returning to Base
l = Require Doctor, Serious Injuries
ll = Require Medical Supplies
F = Require Food & Water
K = Indicate Direction to Proceed
LLL = Operation Completed
LL = All Well
LL (underlined) = All Missing Personnel Found
L = Require Fuel & Oil
_IL (two Ls back to back) = Not Understood
I> = Will attempt takeoff
W = Need Repairs
SOS - International Symbol of Distress
triangle = Probably Safe to Land Here
square = Require Map & Compass
square w/opposite corners open = Aircraft Seriously Damaged

I left out a few: request for firearms & ammo, requesting signal lamp, operation completed.

Other signals:

Distress:
At night, 3 fires in a triangle about 75 ft apart, on highest clear ground (international distress signal) [If you can't keep 3 fires going, 1 is better than none]

During day, 3 smokey fires in a triangle (" "). Green leaves, moss or some water will create white smoke; oil or rubber will create black smoke. Smoky fires only work on calm, clear days.

Setting fire to one isolated tree (do not set a forest fire)

Mirror flashes: aim at planes or the horizon, preferably from a high area.

Whistles - three separate blasts with a pause between each set. Whistles have been heard a mile away.

Gunshots - either in sets of three, or one blast every minute.

If a pilot sees you:

He will fly low over you to acknowledge you.
At night he will flash a green light.
He waggle his wings if he understands your signal.
If your signal is K, he will fly low over you in the direction you should go, possibly repeatedly.

If he does not understand your signal, he will make a right-hand (to him) circle over you. At night he will flash a red light.

You can warn off a helicopter that is attempting to land in a bad place by waving both arms over your head.