Quit smoking for at least thirty days is the most guarenteed way to do this. Try a local head shop and ask the clerk which product he would recommend for ridding your body of the thc quickly. I've heard of products that can wash your system in a matter of days.
The fastest way to get it out of your system is to not put it in to begin with. There is no real way to make it flush through faster then your body can process it. You can try drinking water, taking a shower, it will relieve the symptoms of the exposer, but it will not remove it.
Obviously the fastest way is to make sure it doesn't get into your system in the first place (sorry, couldn't resist lol). There apparently are some teas available in health stores that will assist in safely removing the THC from your system within 12 to 24 hours.