The Workout Plan
For maximum effectiveness, you need to expose your upper back muscles to both thickness and width exercises. Through proper training of the muscles on the outside of the upper back you will achieve the physical look and posture you have always wanted. In order to build up these muscles you will need to perform a lot of pull-ups and pull-downs. Nevertheless, you need to focus on the muscles in the middle of your upper back too, making them thicker through rowing exercises.
You need to organize your workout in three different variants. One is designed for a light exercising session, one for a medium one and one for a heavy workout day.
You can start with your light day, with 15-20 repetitions per set. First, do two sets of close-grip pull-ups or pull-downs, followed by two sets of rows with a barbell, cable or the entire workout machine. Then, get back to 1 or 2 sets of wide-grip pull-downs or pull-ups and do the same number of sets of a different row workout. Finally, finish the day off with 2 sets of various upper back exercises and another set or two of pull-downs, pull-ups or row exercises.
The medium day will be marked with 10 to 15 repetitions per set. Start with 2 or 3 sets of close grip pull-downs or pull-ups with your palms facing you. Then, proceed with the same amount of row sets, followed by the same amount of wide-grip pull-ups or pull-downs. Once this is done, perform a set or two of a different row exercise and finish it off with 2 sets of upper back exercises after which you will do one or two sets of drop exercises.
Finally, your heavy workout day will consist of 5 to 10 repetitions per each set. Start with 3 or 4 sets of close-grip pull-ups or pull-downs, move on to the same number of row sets, returning to 2 to 3 sets of wide-grip pull-ups or pull-downs, only to do 2 or 3 sets of a row exercise and 2 sets of upper back exercises, changing them frequently. End the day with 1 or 2 sets of drops.
Whenever you can, perform pull-ups rather than pull-downs, since your body will respond to these better. Ultimately, you can modify the workout, the grips and many other factors so that they suit you more.