Keep your head on an imaginary wall to hold the arc
Golf is played tilted toward the ground, so the swing arc should be a tilted circle, not flat like baseball. Keep your head roughly against an imaginary wall as you swing and the club will naturally travel on a roughly 45-degree arc. If your head comes off that wall you will feel yourself losing balance, which signals you have come out of the arc.
Channel: Danny Maude
Watch official source at 3:04