Take frequent breaks
According to Wilcox, a break every 20-30 minutes is ideal. Set an alarm or set a schedule for such breaks. One trick is to take a break or at least correct your posture every time you get an e-mail from a certain person or an e-mail about a certain topic multiple times per day. Pause before you open the e-mail; think about your posture and correct it or stand up and go for quick walk around the office before you type a response.