Several golfers allow the left/right knee to collapse toward the right/left on the backswing.
What this does is that it drops your shoulder and consequently sways and overturns your hips.
In order to solve this, you have to imagine your left/right knee going outwards and to the target whilst on the backswing.
It is alright to feel a little tension while this happens.
If you can't visualize it, try putting a basketball or anything similar in between your knees.
Hold it up while doing the swing! The Hitchhiker Method Try visualizing this as you're in the backswing: Place your right hand in a "hitchhiker" position (come on I'm sure you've seen a lot of movies with this).
When the club is near your waist you should be able to visualize or at least look back to see that your thumb is pointing to the sky.
Here's another tip: imagine shaking hands with somebody, with the palm facing neither up or down (sideways).
Right from the start this will make sure that you're positioning yourself correctly before your swing.
The Top Pause Several golfers swing too quickly and this is a very common mistake you see on the driving range and on the course as well.
However this is not to say that you should swing without any acceleration or power at all; rather, it is to give importance to a very important concept in golf and that is to maintain a rhythm.
One thing to practice is to be able to have a slight pause at the top of your swing before shifting gears and beginning the downswing.
Doing this will help you keep your ball in sweet spots of the fairway more often.