As often as I can, I try to work on the thing that I'm most interested in doing at that very moment, even if it's not the most important. Experience has taught me that an hour on the wrong task is better than an hour wasted procrastinating the right one. Eventually my motivation will return to whatever it was that I should have been doing, and at least then there's not a pile of stuff that's accumulated.

As for frustration ... dancing. Seriously. I tend to go out dancing once a week and just shut off my brain, chat with my friends and forget about everything productive for a few hours. It just helps me reset.