I use a variety of GTD-related systems that work smoothly for juggling many projects---as long as I have plenty of energy to pick out the right actions for 'now' and get cracking. However, when I sit down at the end of a long day, I tend to get stuck trying to muster the energy to get back into my system at all.
So, how do you make effective use of time available with "low energy"? And how do you get from "low energy" back into full productivity?