GM actually makes stuff for this. Its available at at Chevy parts dept. Its called "upper engine cleaner"

Or, you could also pour some water in there, small amounts slowly, using a mist bottle. Disconnect intake, run engine. Open throttle plate, and spray in. The engine will bog a little bit, but the water will turn into steam with the heat and clean out the engine. With such a small amount of water, you don't have to worry about any harm.

They used to use crushed walnuts to clean bmw's. They had an attachment that connected to the spark plug hole, and blew air, and the crushed walnuts into it. Then blew the cylinder out with air. Any residual powder that was left burned off when you started the engine.