Deadlifts, rows, and chinups. Deadlifts work nearly every muscle you have, but they're particularly effective for building your traps, which start at the neck and extend all the way down to your middle back. The exercise also adds thickness to your spinal erectors, the cable-like muscles that extend from the top of your glutes to your traps. Experiment with lots of set and rep ranges, but 1-3 sets of 1-20 reps should cover your needs. Rows, particularly with a barbell or dumbbells, will hammer all the muscles around your shoulder blades, including the rhomboids, teres major, and rear delts, which help the upper back look defined. Stick with 2-3 sets of 8-12 reps.