I read a great comment on TM recently, I think it was in a recent article... something to the effect of "you should be able to row as much as you bench." Get your Olympic Skull on, and get on the rowing cable station... FOCUS ON SQUEEZING THE LOW TRAPS (like you're pinching a pencil between your shoulder blades). You should be moving the stack w/ your back, simply using your arms as an extension of your hands to grip w/. Also, in an article yesterday, there was a comment about the "batman" lift: Get facedown on the bench w/ a dumbell in each hand.
Row up to your armpit and hold for 10 seconds. If you want activation in the rhomboids (something I've had trouble w/), YOU WILL FEEL IT ON THIS ONE... how do I know? I did this lift w/ the 10 sec pause for the first time ever tonight. When I left the gym, I felt like I was about 3 inches taller (my shoulders were back, standing taller, etc). Obv. one workout isn't a cure, but you'll feel what good posture should feel like.
Note - before I sat down to row, I did a quick pec stretch (the Eric Cressey version w/ elbow @ 90 degrees, and turn your head away from your arm to accentuate the stretch), followed by placing my thumbs on my low traps and trying to feel a quick squeeze (similar to having someone stand behind you and palpate for the mind-muscle awareness during the lift). Good luck! Also, search "neanderthal no more" on this site and you'll really get some great, professional stuff from that read!