Set your shoulders before each rep. By that I mean actively retract your scapulae. Pretend someone is jamming a hand into your back and you are trying to pinch it with your shoulder blades. Try to hold that "pinch" for each rep. If it comes out as you push the bar up, reset it before the next rep.
Also, stop benching altogether and do whatever shoulder workout that guy is doing behind you. You'll probably have to get a sweet tank top too.