As you swing back, it is important to 'set your wrists' in order to create a lever in your golf swing. Higher-handicappers tend to keep their wrist straight and then end up hinging their elbows instead, creating a narrow, lifted swing. Ideally at the halfway back point, when the lead arm is parallel to the ground, the wrists should be almost fully hinged.

Stand in your golf posture, holding a golf club.
Start your backswing and simply hinge your wrists so that the club turns upside down.
At the halfway back point, when the lead arm is parallel to the ground, the wrists should be almost fully hinged. Stop and check position.
Return to start position and repeat exercise.
Do 8 to 10 repetitions.