I use Time Out app for OS X. Spend some time reading articles about this topic and you'll understand that the most important is to take breaks and avoid long hours in same position. I set it up to lock my screen with postpone button for normal breaks and no button at all for micro breaks. Normal breaks are 2min every 2h and micro 20sec every 30min-1h. While on break I try to move eyes in different directions which is like going to eye gym :D