I've been reading and pondering this topic now for a bit.
Mirror, flashlight, whistle....all fine choices. But how does one go about attracting attention, and indicating they need help if nobody is already looking for them?
In my environment, a signal mirror would only be partially effective for ground-to-air signaling, if the tree cover permitted it.
A whistle has a limited range in a forest, and even then if a person wasn't blowing "SOS" with it, it could be ignored.
Things like bright colored material, again, mainly to mark your spot if being sought from the air.
A blinking flashlight, unless someone watched long enough to know you were flashing SOS, could easily be dismissed as just another hiker on the trail.
Three shots? Really bad marksman. 3 more shots, 30 minutes later? REALLY bad hunter, and stupid deer.
That's why I carry flares. It's pretty freakin' hard to ignore a streak of red phosphorus shooting up out of the trees. If nothing else, the Rangers or Border Patrol will show up to write me a ticket for shooing off fireworks.