Spread 'em Meat Bag!
If you have a gas grill like I do and it tends to run hot, place the burners on the lowest setting, then use the valve from the propane tank to turn the flame down a bit more. If it is windy, you wont be able to reduce the flame much and still keep it lit. In that case just use a beer can to partially prop open the grill lid.
The "proper" way to prep the chicken is to cut out backbone and then spread it flat. My son cut down the breast bone and that worked just as well. Full instructions on the proper method here, but it works either way. Choose whichever technique you prefer and happy grilling!