For regular, consistent productivity (not just the occasional project over a day or two) I have to change my physical environment. This means having an office I go to (or library when I was a student) that--and this is key--is far away enough from my home that it's not convenient to just leave and go home when I'm sick of working. It has to be more of a hassle to go home than to just do another half hour of work (and then another, and another, and so on).