This is one I have to work on myself: Drink more water.

How much should you drink? If you're thirsty, you're dehydrated. You actually need water <em>before</em> you feel thirsty.

When you're dehydrated you feel tired, your muscles ache, your skin gets ashy, you get headaches, and you can get sick.

So drink more because you need it. It's really that simple.