Squats are usually deemed a leg exercise, but they also recruit many other muscles, including your lower back, abdominals, obliques and upper back -- the muscles responsible for maintaining good posture. While squats are a highly effective strength and muscle builder, they shouldn't be underestimated as a way of strengthening your lower back and improving posture.
STRENGTHEN YOUR BACK
Though squats primarily strengthen your lower body, your upper body posture is integral for proper execution; once your upper back starts to round, your lower back will likely arch to counterbalance. Center your upper body over your pelvis, and pull your stomach in toward your spine. Maintain a slightly lifted chest and slide your shoulder blades down your back. Keep your upper back straight as you bend your knees and lower into the squat.
USE YOUR BODY WEIGHT
Using weights that are too heavy for your fitness level can take a toll on your back. Holding heavy dumbbells in your hands or placing a barbell on your upper back that is too much of a load can make it tough to keep your upper back straight and in proper alignment. Your core and pelvis can then be thrown off, causing your stomach to move forward and your lower back to arch. Scale back on your resistance levels if your form is suffering. To improve your stance, you will need to strengthen your core muscles, which include your abs and back.
The videos below are a good beginning. I would recommend seeking a group or personal trainer to help dial in proper form.