Have a pad and a pen for taking notes ready and your GTD Projects and Next Actions lists at hand.
They're for more urgent work that might come up from time to time.
David Allen says the art of personal productivity is a martial art. I consider that
to be a metaphor. In any case, he says it's because you need to be ready for
surprises. Martial arts experts may be ready at all times, being aware of the
distances and positions between themselves and other people during ordinary
times when no fight is going on, so they can avoid being in a vulnerable position
if a fight starts.