Warn your child. Explain that this "game" causes not only slurred speech, gaps of memory, sudden outbursts of anger, but also possible strokes, seizures, retinal damage, brain damage or even death. Be firm and serious in your talks. Most kids have no clue how dangerous this game is or how the brain is effected by a lack of oxygen.
Monitor your child's bedroom. Do so especially if you notice locked or blocked doors and unusual demands for privacy.
Watch for signs. Blood-shot eyes, marks on the neck, asking about asphyxiation. Some children actually think this approach to "getting high" is safer than alcohol or drugs.
Get savvy. Many kids are doing this activity alone for the high. If the child loses consciousness and there is no one there to immediatley release the pressure, he is unable to help himself. The child could suffer brain damage and death usually after three minutes.