.Sit Up Straight.

"Good posture makes you look five pounds thinner"-Charlene Johnson
In order to workout the muscles that affect your posture, you have to exercise your posterior deltoids (back of your shoulder muscles).

Recommended Workout: Bent Over Lateral Raise

Using dumbbells( i don't hold more than 4lbs in each hand), bend over at the waist with your feet shoulder width apart. Keep a slight bend in the knees to prevent stain on the lower back. Hold the dumbbells at arms length in front of you with the palms of your hands facing each other. Keep a slight bend in your elbows.
Using your upper back strength, raise the dumbbells to the back and upwards in a semicircular arc as far as you can. Hold this position for a second to maximize the peak contraction in the rear deltoids. Lower the dumbbells back to the starting position. Repeat until muscle fatigue!!!!