My fire district recently had a training on how to perform a “hot load” (loading a patient into a helicopter while the blades are spinning). Afterwards I had a brief discussion with one of the pilots from Airlift Northwest about locating patients in the wilderness. He mentioned that, during the day, signal mirrors are actually quite effective when used properly but that it’s usually difficult for folks on the ground to shine the light right at the helicopter, which is what actually gets their attention. Some mirrors, such as the UST Starflash, include a sort of targeting reticle which lights up (flashes) when the light from the mirror is reflecting onto whatever’s visible through the hole. I carry what I believe is an even smaller version which used to be sold by Gossamer Gear but their product page no longer exists.
In any case, it’s nice to have confirmation from an actual pilot that these mirrors are effective.
On a side note, they also mentioned that at night their night vision goggles allow them to see light sources from dozens (I want to say he said “hundreds” but that seems like too much) of miles away. A lighter, cell phone screen, or just about anything will be more than bright enough for you to get found.