When I first starting working out, someone once told me that I should treat working out just like I would brushing my teeth. I thought she was nuts, wondering how on earth she could compare the two minutes of brushing my teeth to the 45 minutes of agony at the gym. She put it to me like this: you wouldn't dare go longer than a day without brushing your teeth and you should get into the habit of feeling the same way about exercise.
And after many walks with my dog and trips to the gym, I now get it. Her point was that once you treat something as part of your routine and make it nonnegotiable, thinking about the way you would feel if you didn't do it (imagine how you'd feel if you just skipped brushing your teeth, gross), the more likely you are to do it. So start making being active (even if it's a 15 minute part of your routine), just like brushing your teeth.