Today I rediscovered for the Nth time the value of the three-hour rule*: it takes three hours to get anything "done", especially if that thing is something which requires concentration, like programming. And during that three hours it's best disable all interruptions; don't check email, take phone calls, send texts, etc. Three hours.
A good day includes at least one three hour window, and a great day includes two :) Today ... was a good day. Tomorrow? We'll see :)