Players should ensure that no one is standing close by or in a position to be hit by the club, the ball or any stones, pebbles, twigs or the like when they make a stroke or practice swing.
Players should not play until the players in front are out of range.
Players should always alert greenstaff nearby or ahead when they are about to make a stroke that might endanger them.
If a player plays a ball in a direction where there is a danger of hitting someone, he should immediately shout a warning. The traditional word of warning in such a situation is "fore."
Etiquette courtesy of the USGA.