Re: care of instrument: silver plated 1957 mark vi
Anti-tarnish strips should be your first line of defense. Gently wipe down the silver once in awhile after use and make sure your hands are clean and free of anything foriegn. When it comes time to do a dissasembly and thorough cleaning, use Haggerty's Silversmith products.
Some other products like Flitz work well to take off heavy been-in-the-basement-for-40-years tarnish and crud, but are more abrasive and don't leave as smooth (and shiney) surface as far as I can tell, so be carefull with these.
Silver can tarnish at different rates due to the presense of reactive agents it is exposed to. The presense of these agents will vary by geography a lot of the time. Sulphur is one of the most notable of these agents.