When/if you are offered marijuana, simply respond with, "I'm cool," and raise both hands in a somewhat declining manner. This achieves two things. First, it conveys the message that you do not wish to participate in the activity at this time. Second, by using the phrase "I'm cool," you convey that you are not being negative (i.e., not bringing them down) and that you are "hip."
It should definitely be reiterated that you should under no circumstances take the pipe, bong, spliff, etc. from the person offering and pass it on. As soon as you have the incriminating evidence in your hand, The Man or one of his jack booted thugs will catch you, and you will be prosecuted to the full extent of the law. Simply decline as stated above, and wait for them to pass it in the other direction.
Unless, of course, you want to smoke. Then, by all means, toke away!
This information came from my uncle Tony before seeing the Red Hot Chili Peppers at the Target Center.