The lats and shoulder girdle can get tight and bunched up from a lot of hard training, resulting in adhesions within the muscles that can keep it from "gliding" at its peak efficiency.
Certain exercises are perfect for applying an additional stretch, provided you're careful. This stretching is a great way to create muscle separation while loosening up some connective tissue and soft tissue, resulting in increased shoulder flexibility.
This exercise has loosened up my shoulder girdle more than any other. Stand facing the lat pulldown machine with one of your feet on the bench. Using a close-grip handle, do a sort of row with the weight. Fully straighten your arms and duck your head down at the extended part of the rep. This will feel really uncomfortable in the shoulders the first few sets.
Next, pull the bar in to your mid-ab area and arch your back while squeezing your lats forcefully. You'll notice that you start to loosen up as you go, resulting in a kick-ass lat pump.
Lat Pulldowns with a Forced Stretch
I absolutely love this exercise for upper lat width. You need to make sure you do this right (a good training partner is gold) or you can hurt yourself.
Perform a regular pulldown, but as you go through the negative, let your arms straighten out. Your partner then forces the weight down for extra pressure. He should increase the pressure as your arms get close to being totally straight. No three-second negatives here. Just stretch and then relax at the top for a second while the spotter applies pressure.
Heavy Partial Pulldowns
Use a heavy weight that's going to be impossible to do full range of motion reps with. Pull the weight down to only the top of your head, then let it stretch you at the top. Relax your scapulae and reach. The weight should literally pull you out of your seat a few inches. Again, be smart and don't get sloppy.
Most people do these lying across a bench. Try it this way. Lie on the bench with your head hanging off the end. Lower the weight slowly and only pull up to your forehead. Your lats should get looser and looser with each set, and you'll get the side benefit of some great serratus work.