Ratty has some good points there. Lat pulls, chins and pullovers are good for lat width. Rowing movements do hit the lats but the load is shared more with the romboids, traps and real deltoids. Rows tend to result in thickness in the region. Getting a good squeeze in the shoulder blades and a brief pause at the top of the movement with a nice controlled descent will activate more fiber in the lift. But this applies to virtually any lift. My partner complained about not "feeling it" in his lats when I first started working with him. He speed through his reps and had very little TUL (Time Under Load) as a result. I slowed him down quite a bit on his rep speed. For rows I added bent over shrugs to his back routine. If you are not familiar with them you would lay facedown over a 45 degree bench with a heavy dumbell in each hand. You pull your shoulder blades together slowly and hold the contraction for a few seconds and slowly lower the weight. This really puts a great deal of focus on the mid back and lower traps. For latpulls I had him perform one armed lat pulls for a few weeks. This level of isolation really helped him develop a strong mind muscle link in the lat pull as he would tend to move the weight over the full range of motion thus activating more of the lower lat and obliques. When we switched him back to regular chins and latpulls this really translated well for him as he was able to get a better squeeze in the lats. He told me that the one arm lat pulls were the first time he ever had DOMS in his lats, ever. I suspect that slowing the cadence had a bit to do with that as well.
Our bodies and minds tend to follow the path of least resistance. It is easy to fall into just doing the motions. Sometimes it is better to lower the weight a bit and to really focus on the form and speed of a lift. You will find that you can get more out of a lighter lift with good form and a controlled cadence than trying to accelerate up a weight that is a little too heavy in bad form. Trust me here, you will be able to add that weight back onto the bar in short order. Cheating your form on your reps is really just cheating yourself and wasting your time in the gym. You are there for results right? So why not make every rep count as much as possible? Time under load is one of the best ways to modulate intensity. Try a two to three second cadence and see if that doesnt give you more of the feel in the muscle you are looking for. Count to yourself at the start of the set. Say to yourself "One onethousand" on the way up, pause for a halfsecond, then lower the weight over the course of "Two Onethousand, Three Onethousand".
I know this doesnt help much with your posture issue but hopefully it will address the second part of your concern.
