Obtain a quality glass signal mirror- the 2" x 3" size is sufficient and you will have a signaling tool that can get you rescued in a flash. We have tested such mirrors up to 27 miles in the desert and the record rescue with a glass mirror was 105 miles!
Avoid the plastic or polycarbonate mirrors as well as CDs. These scratch easily and don't have the distance capabilities of a quality glass mirror.
Students are often concerned that a glass mirror is too fragile but in all my years of using and carrying one, I've never suffered a breakage. I store mine in a thin bubble-wrap sleeve to be extra careful.
Here's the beauty in using a glass mirror- they are incredibly easy to accurately flash a distant searcher. Place the mirror up to your eye, tilt the mirror up towards the sun (not directly into the sun though!) until you see a small bead of white light. Make sure the surface of the mirror is not obscured by your brimmed hat or fingers. You may have to angle the mirror a bit if the sun is at an unusual angle in order to see the bead of white light. Once you have the white light, simply move it over to your intended target (plane, searcher on the ground, etc.) and flash the person three times.
A pattern of three is the universal distress signal, so use this whether your signal is three toots on the whistle, three flashes with a mirror, three flashes with a flashlight at night, or firing three shots with a rifle (out of harm’s way).