One really good one I learned from a local yoga program is to sit or stand straight and tall, extend your ams to the sides as if trying to touch opposing walls, then squeeze your shouldr blades together. Pretend there is a lemon or pencil in between the blades, and when you squeeze you are wrapping the muscle around the pencil or squeezing the lemons to make lemonade. Sound silly, but it works.
Also Rhombiod pinches and lat rows are good to develop the back muscles.
That said, you will want to balance out the work you do on your back with front side of the body work.
One thing that has really made me aware of my posture is the Fitprime/Kickbutt series of workouts. One is actually called Flexposture which is primarily a yoga/pilates based workout, but there are a couple of cardio and weight segments, but Tamela really hounds in to stand straight, navel in like a mattress button.
1, 2 an excercise I'll do
3, 4 get down on the floor
5, 6 my shape I'll fix
7, 8 I hate this weight
9, 10 can I do it again?