From Coach Hansen:
The shoulder is a wonderful joint, as long as you keep it working like it should. Sadly, the one exercise that can hurt it more often than any other is one that everyone loves doing, the bench press. If you are having pain in your shoulders during or after bench pressing, I hope you will take some advice from a guy who has been there and done that.
First, stop bench pressing for awhile. Just trying to ‘work through the pain’ is the fast track to nowhere. On the plus side, consider that you will lose a lot less strength from taking a few weeks off than you would from the few months to come back from a full-fledged injury.
Next, if at all possible, go see an expert. Rather than a self-diagnosis, or letting that "one guy" at the gym telling you what is wrong, go to a physical therapist or doctor. They should be able to pin-point the problem and give you the correct exercises to remedy the situation. Please remember, I am talking legitimate pain from an injury here, and not soreness. If you are honest with yourself, you will know the difference.
For the lucky ones who don’t have any pain from benching, let’s keep it that way by following a few simple rules. The first, and most important, is technique. This is probably, to be honest, what got you into trouble in the first place. If you have otherwise-healthy shoulders, and benching is causing pain and/or injury, you likely aren’t doing it right. The "body-builder" style of benching, for example, is a recipe for disaster when trained heavy. I'm referring here to benching with the back flat and the elbows flared, trying to get a stretch of the pecs. This not only puts a ‘stretch on the pecs,’ but a ton of stress on the shoulders. When benching significant loads, the shoulder blades should be pulled together, lifting you off the bench slightly, and the elbows should be tucked into the sides. A moderate arch in the back is good as well. Unless you are competing you don’t have to go crazy with the arch, but a little bit is definitely better than none. Dave Tate has a great video on bench press technique available on his website, Elite Fitness Systems. I highly recommend it, and you don’t have to be a full-fledged powerlifter to watch it. Everyone in the gym wants to bench more, and this video will help you with that. [and don’t forget, it’s FREE: http://train.elitefts.com/exercise-index/barbell/bench-press/ ]
Balance in training is second, and is as big a key as technique. A good rule of thumb is for every set of bench presses you do, a set of rows (low pulley, pulldowns, chin ups, bent over, whatever) should be done as well. I, personally, also recommend you include some overhead pressing based on its carryover to benching. A word to the wise here, however: if you are training the bench heavy, like getting ready for a meet, then don’t train the overhead press heavy at the same time. The shoulders are getting a lot of work from the bench, and heavy overhead pressing can be too much for them. This is why, for the same of variety and balanced development, I recommend periodically taking some time off heavy benching and concentrating on upping your overhead press movements. The results are usually happier shoulders and an increase in the bench due to the shoulder support strength and increased triceps drive. Be sure to pay some attention to the rear delts, as well. Rear delt flys, at a minimum, and face pulls are great for this muscle group.
Having dealt with my own shoulder troubles many times (and losing a ton of training time because of it) I have not only seem my own medical professionals and read the textbooks, but scoured the web for information. There is a lot of it, in fact, so much that it can be overwhelming. I will try to list some sites that I have found helpful, in hopes of saving you some time and grief of your own, in a future post.
Happy benching!