Turn off every alert and notification that you can and disable the clock in your menu/task bar. Get in the zone by not letting external things interrupt you all day. Programmers especially need long stretches of uninterrupted time.

Seeing a clock all day brings you out of the present moment - your mind wanders into useless thinking about how many hours until lunch, or until I go home, or even more negatively, "I've been stuck on this problem for x minutes."