Your setup posture is crucial to hitting solid shots and breaking 100. Weaknesses in the rectus abdominis or glutes can cause either a rounding of the spine or something we call "S-posture," which occurs when there is too much tilt from the pelvis (the spine bends in the shape of the letter S; inset, bottom). In either case, the excessive spinal curve inhibits your ability to rotate toward the target and maintain body angles prior to impact. At address you want to keep the spine as straight as possible, though there will be a slight bend around the neck.
DON'T ARCH YOUR BACK
FILL IN THE GAP TO STAIGHTEN THE SPINE
To check if your spine has too much curvature when you set up to the ball, get into your address posture and hold a clubshaft down the center of your back. If there is a large gap between the shaft and your lower back (more than two fingers wide), you are in an S-posture. Or if you can't lay most of the shaft against your back, you're hunched over. Adjust your posture so the shaft lies flat.
REVERSE CRUNCH FOR STRONGER ABS
Weak abs can limit your ability to keep your spine straight at address. To strengthen, lie supine with knees bent, holding a club over your head. Without moving the club, try to bring your feet up and under the club. This will force a posterior tilt of the pelvis and work the abdominals. Do two sets of 10 reps.