If you have weak stomach muscles, you can build their strength with sit-ups, where you touch your right elbow to your left knee and your left elbow to your right knee. This will help strengthen the abdominals and the obliques at the same time.
Don't initiate your back swing with the stomach. It will throw your timing off. Let your legs start the turn, and then the hips will prompt your stomach/midriff section to wind up away from your target (the area where you want hit your ball).