Set aside blocks of time to work, and during that time, turn off or silence all notifications. Don't keep your email open. Block distracting sites if possible (find a browser plug-in to help you with this).
Keep a minimalist work area. Remove clutter and put away notes not related to what you are trying to focus on.
Put a note about what you are focussing on in an easily-visible place. For example, a post-it on your monitor (assuming it's the only one there, which it should be).
Related to #1, it's helpful to dedicate your computer to work only (at least during working hours). Try using your phone or tablet during breaks to check your social networks, instead of your desktop. Of course, this is after you spend some time looking at something other than a screen.