At the gym, I stick to drills that improve both my physique and my swing. This exercise gets your back into the correct hitting posture while strengthening your legs, a combination that will help you make a power push at impact.
It's easy and only takes a minute or two. Stand erect with your driver held flat against your back—you don't want any curve in your spine. Next, step forward with your right foot and drop your left knee toward the ground. Keep your right shin perpendicular to the ground and your back and club straight. Step back and repeat with the left leg. Do 20 reps daily for three days a week, then copy the straight-back feel when you swing. You'll end up miles down the fairway.
STEP 1 (BELOW LEFT): Stand erect with your back straight. Use your driver to make sure you don't slouch.
STEP 2 (BELOW RIGHT): Lunge forward, keeping your back straight. Building better posture like this pays huge dividends on the tee box.