Wear a heart rate monitor when you exercise. Push yourself, but not to the point that you are in pain. Just remember, exercise should not be easy, but should be performed at a moderate to intense intensity depending on your fitness level. Pay attention to form. Each movement should be done in a slow controlled manner and using the full range of motion.
Correct form allows the muscles to work most effectively.
Get up a half-hour earlier. Exercising first thing in the morning is a great way to start the day. Your workout is done and it leaves you feeling energized. Split up your workouts into two 20 - 30 minute segments. Stay active as much as possible.