I just got done reading a great article called HOWTO: Be more productive on Aaron Swartz: The Weblog. One part I found interesting:
Time when you’re hungry or tired or twitchy is low-quality time. Improving it is simple: eat, sleep, and exercise.
If you’re not exercising, I’d like to encourage you to start again. Please feel free to encourage me next time I forget that exercising actually makes me more likely to get other things done.