Getting a solid 8+ hours of good quality sleep each night. For me this meant:

- Sleeping/waking at the same time each night/morning - Keeping my room cool at night - Relaxation exercises for both body and mind once I get into bed - Getting a comfortable eye mask - Actually taking sleep seriously (read Why We Sleep)

I feel so good each day, I have a better memory and emotional stability, and I don't fall asleep at work any more.