Just dont rub their nose in it - smoke outside if possible, letting them know that youre going OUTSIDE (in the garden) for a smoke can also be a good thing. If you paint or draw/do creative stuff when stoned, show them your paintings and stuff aswell so they can see that its not only something you do to feel good and have a laugh.
Also, if you dont drink (like me) try telling them that its a lot more safer than drinking and there hasnt been a single death as a result of marijuana use. Just a few ideas, Im sure theres loads more that people can come up with.