There's a lot of things that work well. There's throttle body cleaner, carburetor cleaner, brake cleaner, and Berryman's cleaner. All of these are available in a spray can and work pretty well. IMO the Berryman's works the best with the least amount of effort.
If it's really gunked up, you can try setting it upside down on a flat surface and spraying the cleaner into it and letting it sit.
Don't forget to pick up a new gasket, sometimes the old ones are brittle and break easily.