A scenario that I often see is the back leg straightening during the back swing. If your leg straightens then your entire body will rise up a few inches which makes it hard to time the downswing and often results in hitting behind the ball or a fat shot.
What I suggest to fix this problem is to simply focus on maintaining the flex in your back leg throughout your back swing.
Next time you hit some balls focus on your back leg at your address position, keep it slightly bent during your swing and through impact, feeling your weight on the inside of your right thigh.
This will help you TURN around your right hip rather than swaying and / or lifting up.
It will take a few balls to get comfortable with this but I guarantee it will fix the problem.
Read more golf tips on the Bob Krause Golf page on ClickOnDetroit.com.