The lat pulldown is a great exercise for strengthening and developing the upper back muscles that are critical for good posture.  People whose shoulders are slumped forward most likely have underdeveloped upper back muscles.

First, sit down and adjust the thigh pad to a position that firmly fits over your thighs.  Then select a resistance with which the last three repetitions are difficult to complete.  If this is your first time doing this exercise, it may take experimenting at several weights before you find the right resistance.  Next, grip the bar a little wider than shoulder width, sit down on the seat and place your knees firmly under the pad.  Start with your arms fully extended and your chest held high.  This is your start position. Now, pull the bar slowly down to the base of your neck while squeezing your shoulder blades back and together.   Slowly return the bar to the starting position.  (It should take about three seconds to pull the bar down and about two seconds to return the bar to its starting position). Proper breathing is very important, so remember to exhale as you pull the bar down and inhale as you return the bar to the starting position. 

The seated row is also an exercise that targets the muscles of the upper back.  Place the pin in a resistance with which the last three repetitions are difficult to complete, again you may have to experiment to find the appropriate weight for you.  Sit on the bench, and place your feet on the foot rest.  Lean forward, and grab the attachment.  Slide back until your legs are almost fully extended and hold the attachment waist height with your arms fully extended.  This is your start position.  Lift your chest high, and slowly pull your hands to your naval, while rolling your shoulders back and pinching your shoulder blades together.  Then, slowly return to the starting position.  Your breathing pattern for this exercise is to exhale when you are pulling in and inhale when you are returning to the starting position.

I like to incorporate three movements for the upper back because these muscles are often neglected and weak in most clients that I see.  Weak upper back muscles not only lead to posture problems, they also lead to neck and shoulder problems.  The upright row strengthens and develops the top portion of your trapezius -the muscle at the top of your shoulders that attaches to your neck.  Perform this exercise at the cable machine.  Attach a straight bar, and placing the pulley at the lowest level.  Grip the bar at a position where your hands are about an inch apart, and stand upright with your arms fully extended.  Position your feet shoulder width apart, slightly bend your knees, and lift your chest high.  This is your start position.  Pull your hands to the top of your chest, while keeping your elbows slightly higher than your hands. Then return to the start position.  Exhale as you pull the weight upward, and inhale as you return to the start position.