One word. Exercise.
For some girls, it's hard. We have so much to do, where does the time fit in, right? Here's how you work this:
Plan out your day. I have a post on my trueyougirlz.blogspot.com about how to plan out your day. You can go on there to see how to do it. Here is the exact post address: http://trueyougirlz.blogspot.com/2014/03/brain-organization.html
After you plan out your day, decide what time is good for you. When are you least busiest? Morning, noon, night? Choose one, and exercise then.
Have chores? Use chores not against you, but with you! Let's say you had to....mop the kitchen floor every morning. You can play music and dance while your doing your chores. Like, The Rock. (I made this dance move up! You can too!) You rock your hips side to side, when it goes out, make the mop, mop the floor. When it goes in, lift the mop up. Out, in, up, down. It's really fun. When your washing dishes, you can hold the (example) plate, and work out your arms. Hold it with one hand, and the soapy sponge in the other. Make your hands go back and in. Back, in. While your arms are getting stretched, your washing dishes! When you wash out the sponge, make it a finger exercise. Under warm water, slowly make your fingers expand and close around the sponge.
It's more fun to exercise with friends and family. Call up your friends or family and ask them when their free. You can go to the park and have some fun!