I slouch all the time.  I am always trying to remind myself to sit up straight.  I have tried sitting on a medicine ball, sitting on one of those kneeling seats and leaving myself little notes on my monitor reminding myself to straighten up.  I read about this little device in a recent issue of Shape magazine and I think it is so cool.  It is a little vibrator that sits on your back (by your bra strap) that will vibrate a little bit every time your posture is less than ideal. It has a steep price tag $75 (well $85 with shipping) so I haven't purchased it yet (still trying to save money in this economy).  I will definitely write a review when I do, but I still wanted to share!

Check it out at iPosture.