full transparency or 98%
diffuse to 0
eccentricity to something like 0.05
spec rolloff to about 1.5
spec colour to white
if you want a glass ball I'd do the same above then switch on refractions in the raytrace options and tweak the refractive index to something like 1.1 or till it looks good!
also turn up the reflection limit just below the refrac index to something like 2 or 3
the settings of glass will depend on the renderer and what your trying to create. also are you using any hdri reflections/environment in the scene? this will make a difference.
finally the dialetric shader in mental ray creates very good glass too but it can be painful to get your head around