Rough out a character's head at a low clay coarseness. Carve recesses where the eyes will be located. Create a new piece and select the empty workspace option. Resize the workspace so that it is just big enough to contain the eyeballs. Use Add Clay to create two spheres to represent the eyeballs. Keep the eye spheres as a visible separate piece throughout modeling and combine as a last step at a resolution high enough to maintain sharp edges along the eyelids. This also makes it much easier to model the eyelids without carving into the eyes and allows you to see a sharp edge along the eyelid even at extremely coarse clay settings.