Make sure you eating healthy and getting everything you need. Also, remember that in low intensity workouts your body uses fat for fuel, and in higher intensity workouts your body uses the glycogen stores in your muscles and liver. Carbohydrates are what produces the glycogen so I make sure I get enough of those to fill my muscles with fuel. Getting carbs afterwards is good, too, because around 15 minutes after your workout you have the chance to reload up to something like 300% of the glycogen in your muscles.

But yea, make sure you're getting all the stuff you need to fuel your body!