Pilates has an exercise that is designed for posture. It really helps those of us who sit in cars a lot, or those who sit in front of a computer or tv all day.
Sit flat on the floor. Try to put your legs out in front of you flat on the floor, legs together. Try to sit with your back totally straight, trying to make a 90 degree angle with your body. This part will be hard for some people (more guys than gals) who are less flexible. If you find that you have trouble sitting straight up, then bend your knees a little until you can do it.
Now, sitting in that position, hold your arms straight out to either side, level with your shoulders. Take a big deep breath. Now make almost a 90 degree turn to the left and hold it for a good long exhalation. Remember to remain sitting up straight. Picture your back tied to a board. Now move back to center. Take another deep breath. Now make the same turn to the right and hold it for the same length of time. Come back to center. Repeat this for 8-10 times and if you have less than stellar posture, you will really start to feel the burn in your upper back.
There are lots of exercises such as this for posture. I hope this one works for you.
This sounds almost identical to Roman Twists for the obliques, except in that exercise the knees are up.