Having balance in your workout between rowing/pulling and pressing movements is something you should be aware of to start with and is usually the cause. Rounded shoulders is usually a combination of tight pec's and weak upper back muscles - traps/rhomboids. I'm currently doing quite a bit to correct my posture in the same way as you.
To loosen the pecs you can either go with stretching, something like doorway stretches or what I prefer is using a hard ball and rolling my pecs against the wall, this gives me a massive release and looseness.
My next suggestion would be wall slides. These might be hard at first but really make a difference for me when done correctly. Just keep the scaps down and avoid shrugging. Scroll down to number 7 in this article: T NATION | Essential 8 Mobility Drills
To strengthen the upper back, rowing will go a long way, just make sure you're getting the most out of it. Here's some advice from another thread:
Originally Posted by BendtheBar
One of the ways I teach row and pullup form is:
Arms as hooks, start the row by moving the elbows back, finish by contracting the lats.
I use this style in conjunction with explosive pulls. It can often be misinterpreted as a bodybuilding mind-muscle concept, but it's not presented as such. It's mostly for training myself to use as much back as possible during my rows, so I don't pull only with my arms.
To really target the upper back and rhomboids I like to do band pull-aparts but there are a few other exercises you can do, diesel crew on youtube have a lot of videos for these. Face-pulls is a favourite of mine also with a scapula retraction at the start of each rep like in this video:
The last thing that I add into my upper body stretch/mobility routine is thoracic extensions. You can either use a foam roller or 2 tennis balls taped together for this, I think one version is in the T-Nation article linked above.
Doing these regularly and properly should start to make a difference and even just forcing your shoulders back when walking around will make a difference. Hopefully this will help you and if there's anything else I'm sure others will chime in.