4. Using the wrong shafts
A vastly underrated proponent of the golf club. Many zone in on the head of a club, and forget about the shaft. Golfers will not be tapping the full potential of their bats if they do this.
Do you need graphite or steel? What weight? Flex? Should you be playing a shorter driver length shaft for more accuracy off the tee?
High handicappers may not believe it will make a big difference to their game, but will be stunned when the correct shaft is attached to their clubs.