The only thing that I've ever tried that worked for me, was giving them
a bath in a sink with warm water with a little "mild" soap. Then gently
rub them down with a Q-Tip followed by a good rinsing. If I had some
that where stubborn, I'd "gently" rub them down with original Crest
toothpaste keeping them very wet. Some I've even followed up with
either an automotive polishing compound or just a good paste wax.
I should mention that my cars were painted with enamel. Ones with
decals, you don't want to soak in the bath too long or rub too hard.
If there's an easier way, I sure would have liked to known about it.
I'm planning on glass display cabinets from now on.