Diet has made a huge difference in maintaining my energy levels throughout the day. In the morning I make a vegetable + fruit smoothie, lunch I usually eat plain chili or some form of meat and vegetables. I try to reserve carbs for before/after exercise.

On days when I splurge and eat pizza or some heavy carbs for lunch, I usually crash pretty hard around 2pm. That's why I save lunch beers for Fridays only! :)