Buying race cut Euro stuff (Castelli and Giordana mainly) helped me. I'm fairly thick from the waist up, although I don't have big arms so I'm not quite in the same boat as you...but I've come to like the tight fit in the shoulders, particularly once I'm on the bike. I've also come to terms with wearing very tight jerseys, particularly since it really does eliminate the sag issue. IME, if it's not tight, it's going to sag no matter what.