Look at most people who train and their back will almost always be their weakest body part.
It makes sense. We don't see our backs.
Unfortunately for most, everyone else does.
First and foremost, train your back just as you train every other body part. We are currently training five days a week -- Chest, Back, Legs, Arms, and Shoulders (with abs and calves mixed in two of those days each week.) Notice back is trained just like every other major body part.
Here's some good exercises for you to try -- keep it fresh and you'll keep it going:
Incline dumbbell rows. Lie face-down on an incline bench and row two dumbbells up, rotating your wrists from an overhand grip at the bottom to a palms-facing-your-sides grip at the top.
One-arm pulldowns With one hand. Grab a D-handle attached to a pulldown cable. Pull the handle down to the side of your chest (armpit area). With a longer range of motion, it’s easier to place more emphasis on the contraction than in a standard (two-hand) pulldown.
Seated cable high rows. With an overhand grip, grasp two ends of a rope attached to a low cable. Pull the rope back and up and separate the two parts so that at the top position, your hands are near your face but at opposite sides. This exercise combines a low cable row with an upright row; it works the lats and the lower and upper traps.
High-cable arms-out pulldowns. Stand in the middle of a cable crossover station and hold D-handles attached to two high cables (your arms will be out and up). Pull your elbows down and to your sides at the same time. You should feel a strong stretch in your outer lats at the top and a firm contraction in your inner, lower traps at the bottom position.
Train hard; diet harder!