I'm frequently described as energetic, although there are plenty of mornings where I am, as The Happiness Project's Gretchen Rubin would say, acting the way I want to feel.
Over the years, I've figured out a few ways to maximize energy:
Know your temperment. When I do my best to put out energy, I get back lots from the people who interact with me, which then gives me more, since I am an extrovert. A few years ago, I learned a distinction that rings true: extroverts get energy from being around others, whereas introverts build up energy reserves by spending time by themselves. (If you're unsure about your own temperment, try this quiz.) Because I am an only child I can retreat when I am feeling overwhelmed, but I've learned that once I overcome this and call a friend or two and see them things are a lot better. From the other perspective, if you find being around others exhausts you after a while, don't feel badly about turning down a social invitation.
Figure out what gives you energy. Blogging properly takes lots of time, and I didn't start because I don't have anything else to do. My life, like most people's, generally feels a little too full. But much of my day involves scheduling, lists, making sure things get done. The time I spend doing creative things (and writing is my favorite among them) gives me more energy because I get "flow" and let go of my daily worries while I am engaged in the activity. Determine which activities give you that feeling, and make time to do them.
Understand what drains your energy reserves. Work is called work for a reason, so of course it's going to use up energy, but if you hate what you're doing all day, every day, it's much worse. Are there things that wipe you out that you can control? A long commute is not so bad for some people (I actually love to drive and don't mind traffic, so mine isn't as bad as it might be for others) but for others it's torture. I get overwhelmed by a to-do list that's too long, even if the things on it are small. So I've realized that taking care of a bunch of little things gives me the same sense of straightening up my house. I don't have to dust the baseboards (literally or metaphorically) but staying mentally tidy gives me a sense of calm.
Know who drains your energy. We all have them--friends, co-workers, family members--who bring us down. Energy is contagious, but so is pessimism. Sometimes the glass is just half-empty for these folks, but other times we let them drag us into their personal dramas. This is good for them--that way, we own at least part of their mental baggage, which maybe makes their load lighter--but we have our own bags to carry. So be supportive, but make sure you're not the caddy. As for the truly toxic folks, unless they gave you life, tell the truth and run.
Schedule rest. Don't wait until you bottom out to take time to rest, or your health will inevitably suffer. Block off time to do nothing, and apologize to no one. If you've got children and a spouse, make sure they know that when the bathroom door is locked, you're in the bath. When you take off to play a sport, make sure your phone is off. If you've got dear friends who live in other cities, get on a plane and go and see them. And don't feel guilty. That's the biggest energy drain of all.