Holding the correct posture during the usual forward crouching activities that are a hallmark of the average person's day is one of the most difficult things to keep in mind. Mindfulness is the key here. There are exercise upon exercises that can be performed for improving posture, but the one element that is in my opinion most crucial, yet most often neglected, is simply being aware of what your shouders, head, upper back are doing. Being mindful of the present. At any given moment hundreds of core muscle fibers are firing at one time to hold the position of your body. Just be aware of this. When you can connect your mind with what you body is doing and correct yourself when neccessary you'll find your posture, and surely your well being, increase.
So I play this game when I am in any moment. I focus on my breathing and what it is I am doing at the moment for let's say 15 breaths. I connect each breath to my shoulders. What I mean by that is, with each breath I take, I am thinking about keeping my shoulders and head back. I work to increase the amounts of breath I stay mindful for ( 15 progressing to 20, 25 and so on), and after at time it seems to be more automatic.
While doing exercises to strengthen, lengthen, and create balance among the muscles of the scapula, shoulder, and anterior portion of the body ( pecs and anterior deltoids) is undeniably important to creating better posture, it is the holding of these positions while not lifting weights that will keep your posture nearing perfection for the long run.