I have been working out for the better part of 14 years now and have only recently discovered about a year ago that I was never really working back that hard.....I now am getting the form down (retraining form) while working back to make sure the motion starts with shoulders down and blades pinched, chest out and abs tight and really contracting the back muscles at each rep....but what about while working shoulders, chest, bi's and tri's - should you maintain a 'shoulder down and blades pinched, chest out and abs tight' form while working chest, shoulders, bi's and tri's? Due to a pain in my left shoulder blade due to a muscle imbalance, a trainer always emphasized with pretty much any movement, "Shoulders down, blades pinched, chest out, abs tight".....I have now become accustomed to this form while doing just about every exercise but I seem to experience some shoulder pain while working chest (press, incline, fly) while maintaining this 'proper form' (i.e. shoulders down and blades pinched, chest out and abs tight) - is this form correct for all upper body lifting!?!? Any thoughts??

Shoulders down and back will be optimal for bench. It looks like the problem comes from attempting a harsh move like inlcine fly. I would advise you either get alternate exercises or limit the range of motion so not to overemphasize the shoulder muscles.