The winter time is perfect for making posture and swing changes. I’m a firm believer that you CAN change your swing without a club and ball. What you need to do is change the sequence and way the body and arms work and move, and all you need is a mirror and lots of reps.
Most golfers don’t have good posture–find a picture both face on and down the line of Adam Scott, Tiger, Rory, any of the good players to mimic. You need sharp angles, a straight back, weight over the front of your ankles–not on the balls of your feet and spine tilt to the right (right handed golfer) face on.
I love the DEAD drill from Chuck Quinton (Rotaryswing.com) on You Tube. DEAD stands for Drill to End All Drills. It starts out by helping you understand how to get wound and loaded on the backswing, then the proper way to shift the weight, finally posting of the left leg which clears the left hip. You start doing this with your arms crossed across your chest, then you add the lead arm, then the lead arm with a club upside down and finally both arms and club right side up.
Work on the posture and DEAD drill in a mirror during the off season months and you’ll be amazed the changes you’ll see when the weather breaks in the spring.