One of the biggest holes I see in most athletes development is usually upper back strength. I feel that your upper back is one of the most important muscle groups you can work yet it's so highly neglected by so many people. Why? Maybe because we don't know how to work it? Maybe we just don't see it as that important? I think that if you really look at your development and you're honest with yourself you'll probably agree that your upper back could use some extra attention.
Are you one of those people that can pull just about any deadlift off the floor but the trouble is always in the lockout? Or when front squats get heavy do you start rounding over in the hole often dropping the bar? These are signs that your upper back needs to be stronger. In my opinion the most important thing that your upper back does is provide the stability or the "uprightness" needed to move big weight. By this I mean keeping you upright in the hole on a squat, reigning the bar in when a deadlift gets a little in front of you, and providing a big stable platform off which to press, whether it be bench press or overhead pressing. The upper back should be activated in almost every single movement you do.
Now, I know some will say they train their upper back, after all they knock out a couple sets of pull downs or pull ups at the end of the workout. This is a half assed approach to training your upper back. I feel the upper back should be trained with heavy compound movements just like you would train any other area for strength and performance. We all know isolation exercises like leg extensions don't do shit for your athletic performance, it only makes sense that pull downs won't either. If you want a big strong upper back that will increase your lifts you have to treat training it with the same respect and approach you do all the other body parts you want to get huge and strong.
I am a firm believer in using variations of exercises you already use in order to increase your upper back strength. I believe in using variations simply because typically learning the form of the variation will be much easier than learning something completely new, also the use of variations makes programming upper back work into your routine very easy. So what variation should we use? For me personally I have seen the most results from using the hang or muscle variations of the Olympic lifts. Muscle snatches are my favorite, followed by hang snatches, then muscle cleans and hang cleans. I prefer these movements because they increase the explosiveness of my upper back in addition to the raw strength. There are additional movements you can use though if you're not adept at the Olympic lifts. Bent over rows and t-bar rows can also be very useful. Whichever movements you decide to use the main thing to remember is to train them for strength, which means lower reps and heavier weight. Don't flippantly add 3 sets of 8 bent over rows at the end of your workout, this is not how you would approach squats, and if you want a monster squat I suggest you take your upper back just as seriously.
Keep in mind that your body works as a unit, every muscle group is important and vital to your overall gains. Treat it as such and you'll see more progress than you've ever seen.