Exercise. Make it your goal to exercise for at least one hour a day. This does not need to be all at once. When exercising, make sure you are getting your heart rate up for at least ten minutes at a time. Here are some example exercises (try to do all three every day):
Do 2 minutes of planks, 4 minutes of jumping jacks, and 4 minutes of squats when you wake up.
If you have time before getting ready for work, go for a half hour jog.
Use a stationary or mobile bike for half an hour when you get home.