To get the maximum benefit out of your workout you want to pay close attention to your posture and form. You can work a very different muscle with bad form.
Try to take it slow and controlled when working out. Use the mirrors to your advantage and check your form. I like to tell my clients to squeeze their shoulder blades together, as if you are squeezing a walnut.
The shoulders seem to be the biggest issue with bad form. Many people's shoulders tend to role forward while working out. If you keep your chin up, the chest will follow and it helps to concentrate on the muscle you are trying to exercise.
Your core stability will also improve by engaging the abdomen muscles instead of using your lower back.