Sleep: I think we undervalue the importance of sleep. This is the time when your body gets to improve itself. Without good sleep, your diet and exercise will be much less effective.
Diet: Your mental and physical well being is a rolling average of the things you eat. Cheat days are fine; cheat weeks are not. As long as your diet is always trending in the right direction, you'll find life becomes a little easier to manage at an increasing rate.
Exercise: I hardly do it for physical reasons (though they are very positive), but instead for chemical reasons. Exercise is very important for your brain, and to flush toxins. I play soccer and lift weights, but you might find something else that you enjoy. The important thing—for me—is that it's something you can get into a "flow" with, that feels almost meditative.
Reflect: Whether you do a 5-minute journal, transcendental meditation, 'Getting Thing Done', or whatever system is popular today... it's important to find where your next 2% improvement will come from. Life is just a series of these steps that push you forward, 2% at a time.
Don't forget to play, either. That's half the human experience (and the source of a lot of creativity).