You could try using Argentium silver instead of regular sterling.
Or you might also consider a product Rio carries called Midas
Tarnish Shield, which, after a 10-minute application, leaves an
invisible film on silver to help prevent tarnish. It's not permanent,
but ought to keep your items shiny for quite a few months.
Tarnish Shield is principally perchlorethylene, which is a bit nasty
and requires a respirator. You will also need an acid dip (sold for
plating applications), and, of course, an ultrasonic cleaner.