Espm has good advice. Follow it. But, I will tell you a few things that helped me in addition to that.
In a row, concentrate on squeezing the shoulder blades together, rather than lifting the weight. The weight is irrelevant, it's the motion that works the upper back muscles. Also, try to set up and hit the lower chest/upper stomach because that engages the Lats more. You're pulling the weight in to your body, not snatching it towards your chest. If that makes any sense.
Also, a very good upper back strengthening exercise is to simply sit upright, draw your chest up and shoulders back in a somewhat exaggerated arch of your back, and hold that position while you breathe at least a few times, trying to draw your chest up to your chin. Work up to 45-60 sec. in that position.
You could also do some rounded back good mornings , seated or vertical. Bar on your back, slouch down rounded back, then arch back and raise body into perfect upper body positioning, and raise back up. I do these every pre-workout warmup, and a deloaded version post workout everyday, except weekends.