It is important to take a break regularly to prevent fatigue (physical, mental and visual).
Try to vary and alter the tasks you carry out as a change in activity also offers a break.
Without a doubt, breaks away from the desk are best but a quick stretch whilst seated is better than nothing. Breaks could include standing briefly or walking to the printer.
Remember to take visual breaks too. Try the rule of 20's: every 20 minutes look 20ft away for 20 seconds.