I've been told by more than one rescue pilot that if you are merrily hiking along and a helicopter flies in close, you and everyone in your party should face the helicopter and make the "N" body signal with your arms to signal "No". Don't wave your arms - just stand there making the signal. This tells the pilot that you do not need help and they can fly off to look for the hikers they are looking for.

Of course, if you do need help, then everyone in your party should raise both hands in a "Y" for "Yes" (again no waving - just make the signal, face the pilot and stand there) and the pilot will interpret that as you needing help.

Pretty simple stuff.