SOS is still useful in some situations, esp if some older-timers or ham radio operators are looking for you. Don't underestimate the Hams that will respond to an emergency.
Dial your cell phone occasionally, even if you can't get through. If authorities are tracking your pings, they will know you're still alive and likely continue the search. Once again, don't go crazy and wear down your battery. (You charged it before you got yourself into this predicament, right?)
SOS is ...---... (dit-dit-dit dah-dah-dah dit-dit-dit) using flashes of light (flashlight, headlights) or a whistle, car horn, one of those personal alarms, or an air horn: 3-short+3-long+3-short, 1-minute pause, then repeat.
If your signal source requires batteries, don't panic and wear them out in the first 20-30 minutes. Respond to lights, shouts, movement, vehicles, low planes (not airliners that are crusing six miles high) and helicopters, and noises that could be signals of people looking for you.
If all you can do is beat one rock with another, use 3 quick but distinct taps, then 3 slow taps, then another 3 quick taps (tap-tap-tap tap..... tap..... tap tap-tap-tap).
If you can leave a message in a open area, like desert or beach sand, just spell out HELP or SOS (SOS is read the same way upside down as it is rightside up ) by digging trenches to spell the WORDS.
Remember that CONTRAST is your friend. If you dig trenches in sand or dirt, pile the excavated sand right beside the trench to make shadows formed by the furrows and hills, or ...
Dig the trenches and fill them with brush, or dark rocks on light sand, etc.
Make them fairly large so they can be seen from the air. (You've all seen those Lost Dog or Yard Sale signs written in 1/2" letters where traffic going 50 mph, right? Don't do that.)
Pilots can see movement, so wave your arms or t-shirt or jacket. Yelling at aircraft is only done in movies.