A reader of Marginal Revolution has provided Tyler Cowen (and now Jason Kottke) Walter Benjamin's rules for writing. I am most struck by this sequence:
VII. Never stop writing because you have run out of ideas. Literary honour requires that one break off only at an appointed moment (a mealtime, a meeting) or at the end of the work.
VIII. Fill the lacunae of inspiration by tidily copying out what is already written. Intuition will awaken in the process.
IX. Nulla dies sine linea -- but there may well be weeks.
All of these are variations on the simple theme that dominates all useful advice about accomplishing long-term tasks: don't stop.
Every long-term project will see cycles of productivity as inspiration, competing demands, and incoming resources vary. In other words, everybody slows down. And as Benjamin's ninth rule jokingly hints, pretty much everybody stops, too. But when somebody keeps the low points in the productive cycle just barely above zero, then takes the high points as they come, stuff gets done. When just barely above zero falls to zero, and zero starts to feel maybe a little OK, it's over. I've experienced both sides.
Moral: control your low points for long-term results.