Always be sure to properly fuel your body. A lot can be said about the specifics on how to do that (I recommend reading Eat Smart, Play Hard for those very specifics), but the easiest thing to remember is to eat before and after you work out.
If you eat before you workout, then your body will have the fuel for a better workout, and you will most likely burn more calories than you would on an empty stomach. If you eat after your workout, you will recover quicker, be less sore, and will most likely keep your metabolism chugging away at a high rate for a longer period of time, which also leads to more calories being burned.
Experiment to see what works for your body. A bagel or some toast with juice can often be enough for me before lifting weights or going for an easy run, but for harder workouts I will usually have something a little more substantial such as eggs or cereal. When I finish lifting, I usually eat a protein bar and eat some eggs. A breakfast or granola bar would be fine. After a run, a bagel usually goes down well for me.