Don't half work on something while browsing HN. If you are going to not work, might as well actually gain the benefit of a break. Try to make it so you are all way "on" or all the way "off". Have a cue that separates the too. Some people have a work computer and a play computer. Personally it's enough to have two different desktop environments that I switch between. If you find yourself lingering in the 'off' state, get up and take a walk outside.

Basically train yourself to notice when you are wasting time, but also forgive yourself for not being able to work 12hrs a day every day with absolute discipline.

Edit: Internet research is dangerous and tends to fragment attention. Batch your searches. Stop yourself from instantly diving into a google search for the details of whatever problem you are working on. Instead have a little notepad of things you need to check on later. Try to distill a narrow question that you are trying to answer.