One general fitness tip that will help keep your energy level high and in balance is to make sure you "deload" (rest and don't exercise as hard) for a few days after an intense training session. This will give your body time to recover and prepare for the next set of training demands that you will make on it.
Eat a carbohydrate heavy meal before participating in an invigorating exercise routine. Carbohydrates will fuel your body and provide you with the energy you need to get through your workout. They will also help you to feel satisfied. Eat your meal about thirty minutes before beginning to exercise.
A counter intuitive tip when it comes to fitness is to not overdo it. Perhaps you don't have the same problem as most people and you are super motivated and you go to the gym nearly every day. This is great, except sometimes you may push hard, end up injured, and not getting in any sort of fitness related activity.