The only proven way to beat a breathalyser is to remain sober. There are many theories, but none have been proven.
The most logical theory I've heard is to hyperventilate, to theoretically replace as much of the ethanol (what is measured) with oxygen as possible, into your lungs prior to the test. However this will probably just make you look even more drunk, due to the fact hyperventilation shares many symptoms with intoxication (ie lightheadedness, dizziness, slurred speech).
For the charcoal theory to work you would need to filter ALL the air coming out of your lungs through the charcoal (to filter out the ethanol). It would be like trying to blow up a car tyre with your mouth, in other words it would take more pressure to push the air through the charcoal then average lungs could achieve.
