If you're starting to get back pain, you might need to exercise more. Consider a walking or biking routine, and some back exercises[1].

Also, set an alarm to get up from your desk once an hour. Just taking a trip to the bathroom or the water cooler will help keep you active enough.

[1] I do this routine a couple times a month: http://www.mayoclinic.com/health/back-pain/LB00001_D

(Building some lower-back strength with deadlifts would be the next step, but might be more than you need just to avoid pain.)