I, myself do not smoke marijuana but it is a lifestyle choice that many fall favor to and it not necessarily wrong, so far it has been proven that the active drug, THC, or "Tetrahydrocannabinol" can cause brain damage and mental illness for certain members of the youth community in our society today. On that note the smell of marijuana is quite strong so there are a few ways to cloak your habits.
1. You can smoke in your boxers and wear a muscle shirt, then change and wash those clothes with a strong smelling soap, then shower after thoroughly and brush your teeth, and use a deodorant of some kind.
2. Smoke and completely change your clothes and wash them, and shower (if you're really that cautious), use deodorant, and then wash the clothes and brush your teeth.
The two methods I suggested above are about hiding the smell and odorous evidence that is produced by smoking marijuana, so by washing your clothes you remove the smell from what you were wearing while smoking, and also changing the smell of your breath in order to avoid detection when conversing with members of your family and etc.
Another final option is that you could just not smoke when you're with your parents ex. they go away for a vacation or short trip that lasts over a day, just make sure you don't smoke inside the house or anywhere with dry wall because the material can actually absorb odors that make contact with them.
Good luck!
Here's the article on Tetrahydrocannabinol (for a overall rundown of what it is)
http://en.wikipedia.org/wiki/Tetrahydrocannabinol
參考: I used to clean peoples houses and I had to remove stains and smells and etc.