I have found that doing a lot of dead lifts at the gym, with correct form (straight or concave back), helps me to understand how my spine should be curving. I find that correct form from dead-lifting carries over to sitting, standing, and even jumping, landing, batting a baseball.
I've found that working my lower back muscles helps me sit up straighter. For my slouch, working my traps! and lats helps pull my shoulders back.
Sitting on things like unstable stools and exercise balls also help with posture, assuming that your workspace is set up to ensure you remain upright and don't slouch.
Sorry, I just can't help myself, but in addition to the great suggestions above, the Opedix Posture Shirt may be just what you're looking for:
http://opedix.com/ps_posture_shirt.aspx
It pulls your shoulders back and helps you stand/sit taller. It can even help those with shoulder and neck pain due to poor posture. I wear mine whenever I need to look more impressive or my neck hurts a little.
Full disclosure, I do their PR, but I really do use mine regularly and my coworkers will tell you I'm a happier person when I do.
Being aware of your posture is a large part of the fight to avoid hunching over and staying that way, but there are some other things you can do to help, as well.
A drama professor in college taught me this exercise to improve spinal alignment and to decrease slouching: standing up straight, allow your head to droop down toward your toes while keeping your back straight. Then, slowly begin to bend forward, allowing your head to drag your body forward, and try to feel each vertebrae as you gradually bend to a 90 degree angle. As you get lower, allow your knees to bend a little (this isn't a hamstring stretch) until you're all the way down and then just take a few breaths. Reverse the process until you're upright and I guarantee you'll feel less stress in your back and notice better posture immediately.
Also, you can look into a kneeling desk chair. You may look a little silly sitting in one, but they're excellent for maintaining perfect posture and you're coworkers are going to be buying themselves some after they try yours out.
Other than that, constantly keep your posture in mind and don't make concessions. By concessions I mean don't slouch because you feel it socially awkward to have correct posture. Many times, people slouch in situations where they feel they look odd maintaining good posture (sitting at the dinner table, watching TV, etc.) but never mind that. Always remember to sit up and stand tall and people will come to respect you for your stature and not belittle you for caring about something that affects your health.
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I have found that doing a lot of dead lifts at the gym, with correct form (straight or concave back), helps me to understand how my spine should be curving. I find that correct form from dead-lifting carries over to sitting, standing, and even jumping, landing, batting a baseball.