A common swing tip for the new golfer is that they should take the club straight back along the target line to start the backswing. This is simply not true and can cause many swing faults for the player who attempts to start their backswing in this way.
Firstly the move can result in a lateral weight shift rather than a coiling of the upper body over the right hip. In moving laterally the weight will move to the outside of the right foot and a loss of balance occurs. Such a straight back move can also result in a disconnection of the arms from the body. The arms now swing the club separately from the body and rather than them working together to produce maximum power, they work independently and power is lost. A straight back takeaway can also promote a closed clubface making it easier to hit shots to the left of target.
The golf swing works in an arc, where the center of that arc is very close to the center of the chest between the two shoulders. As long as you have two hands on the grip then this will be true of every golf swing. Thus there is no way the club can be swung fluently and work in a straight line at any time during the swing. Our goal is to maintain width in the backswing and allow the club to stay on plane for as long as possible. An on plane swing is one in which the shaft of the club is either pointing at or parallel to the target line at all times. The more time the club is on plane, the more consistent your shot making will be.
So your goal is not to take the club straight back along the target line, but rather to move the club away on plane. A great drill to help you improve this area of your swing is to use a mirror and your 5 iron.
Take your set up with a mirror to your right side and holding the 5 iron at the bottom of the grip. Place a second club on the ground to represent your target line. Now slowly make your takeaway while watching your movements in the mirror. Your goal is to ensure the club you are holding is always either pointing at or parallel to the club on the ground at all times. When you can do this, you are swinging on plane.
The next step is to take your normal set up and repeat the exercise until you can perform the action at half swing speed and a three quarter backswing and follow through. Then you can tee up a ball and hit a few shots being sure to focus on the movements and not react to the ball flight. When you can keep the club on plane the ball flight will start to become more consistent.
This drill is one you can always go back to when you feel your swing getting off line. It will reinforce good swing mechanics and is simple enough to do anywhere. It is a drill that you just can not spend enough time repeating and is one that should be in every golfers practice regime.