Don't smoke every day.
Do it only on weekends. If you start toking in obv spots like your backyard and your room, your chances on avoiding being cought are really slim.
Go to friends' houses, go to the park, or just anywhere far from home.

When I started to smoke regulary I just fessed up to my parents right away and they were actually really cool with it. However, the fact that I'm 18 played a large factor. And the fact that I'm one of the best performing students in the school prolly played a role too. hehe