I've had my silver Z tenor for two years now and it still looks pretty good. With semi-regular polishing (with a silver-finish cloth, one is included) the finish holds up pretty well, especially if it is kept in the case when not in play.
Over time, I think most finishes tend to naturally lose some luster, and there isn't a whole lot that can be done. I believe that using any kind of liquid or gel tarnish remover would probably damage the finish rather than help it. More experienced techs could chime in on this.