Bon Ami + newspapers + elbow grease.  I am not sure if Bon Ami is still
available in cake form, but that is what my Sweetheart used to use.  She would
apply it with a dampened sponge then rub like hell with crumpled newspaper, looked
a little like the Tasmanian Devil in a storm of white feldspar dust.  She did
this in our first apartment with 9 over 3 double hung windows.  The glass
would sparkle, but after that experience she has since said, "Screw that
bullsh!t!"

George