In Chinese medical theory, martial arts, philosophy, etc. each person has two basic types of energy: pre-birth and post-birth.
Pre-birth energy is literally that which you were born with - it's the energy levels inherited from your parents, grandparents and the rest of your ancestral line, much in the same way that you inherit some of their physical features and psychological characteristics. The amount of pre-birth energy you have is pretty much set: you can't do much to change it.
Post-birth energy is that which you take on from external sources and generate yourself during the course of your life. This energy is derived from the air you breathe, the food & drink you ingest and the exercise (both physical and psychological) that you do. This is the energy that you can add to and improve.
I won't labour the point about exercise as we all know how it works: you put the body under slightly more stress than it's used to and the stress causes some minor damage. In the recovery period after the exercise your body repairs the damage and reinforces the affected areas to withstand future stress. That's how muscles get bigger, heart and lungs get stronger, circulation improves, etc. It's also how your energy levels rise: as your body gets accustomed to the greater levels of stress and improves its overall function, 'normal' activities become so much easier in comparison that you feel more energetic.
Air is by far and away the most important one (anyone who disagrees is welcome to stop breathing for 10 minutes and get back to me later) and is usually the one that gets ignored. It's very difficult to get into the habit of breathing properly - only a few well-trained people actually manage it. Singers and people who play wind instruments do it very well as the ability to use their lungs & diaphragm properly is the foundation of their performing ability. Most of the rest of us don't even think about improving our breathing we just get on with it. However, using the whole of the lung while breathing keeps it in good working order (use it or lose it!) and even if you only do it for a few minutes a day it makes a difference. [To digress for a second: deep breathing improves your circulation, especially if you're a desk-worker. When you're moving around the heart pumps blood through the arteries and the contract/release motion of your muscles pumps blood back to the heart. When you're sitting still the muscle pump isn't working so it's the pressure of the 'fresh' blood that forces the 'old' blood back to the heart. If, however, you breathe deeply into your abdomen, the whole of your torso becomes a pump and helps move the blood around. Stretching the diaphragm like that also helps release tension in the upper body which, as the solar plexus is the emotional centre of the body, helps you to relax. On the flipside: deliberately relaxing your upper body improves your breathing as well as your posture.] If you need any more convincing about the importance of the air you breathe, try taking a walk in the countryside on a warm sunny day and whenever you get to the top of a hill just take a minute or two to deeply breathe in the pure clean air and see how that makes you feel.
Now for the one we have the most control over and the greatest obsession with: food. Think about the old adage that 'you are what you eat', well that is literally true. You eat an apple (or whatever), it's dissolved into liquid and the constituent parts separated out - some of those parts are used to create new cells in your body. That should demonstrate how vitally important food is to your general health. As to your energy levels; because of the sheer complexity of turning apples into muscle, the digestive process actually uses up an awful lot of energy. The purer the food you eat is, the easier it is digested and therefore the less energy it uses up in the process. If you ate nothing but lean meat, vegetables and brown rice you'd have a digestive system running at warp speed and maximum efficiency at all times. The minute you eat or drink anything impure your body has to work harder at digestion. That harder work slows everything down and uses up even more energy, which is why you feel sluggish and sleepy after big meals, especially if that meal happened to be junk. One of the best things you can do to improve your energy levels is to eat less junk. Now, what constitutes junk for one person may be Mother's milk for another and it can be very difficult to tell what's what. For example, when I had my first food intolerance test the results showed that I shouldn't eat celery or soya, both of which came as a surprise, as well as the usual admonition about sugar, dairy, wheat, yeast, caffeine and alcohol, which were basically to be expected. When your digestive system is working efficiently it uses up less energy while processing and because your toilet visits are more regular you're using up less energy generally because you are literally carrying less crap around with you. If you're using less energy for one thing, more energy is available for everything else - it really is that simple.
[The caveat about improving your diet is the bit they usually don't tell you: during the first 1-2 weeks while your body is adjusting to the change you will feel like hell. Caffeine withdrawal gives you headaches, sugar withdrawal causes mood swings, you'll be hungry most of the time and constantly thinking about your next meal, the food you are allowed to eat will seem bland to the point of revulsion and you'll be incredibly short-tempered.]
The overall point is this: improving your energy levels using physical means is best done in the good old-fashioned twin-pronged diet and exercise approach. I'm gradually improving my diet and when I've shifted this awful cold my workout routine will go from basic to brutal in a very short space of time but will still leave space for my increasingly important Taiji training.