Very carefully. You can't be aggressive. At all.
The older (500-S, TA-600, X-101, etc.) have that smooth lacquered finish over the brass metal. We're all familiar with how the finish starts to deteriorate, and then the spots of tarnish begin to appear.
Here's the main problem...that tarnish is underneath what's left of the finish. If you try to remove the tarnished spots, you'll remove what's left of the finish, and then you'll have a shiny spot, and you're hosed.
All you can do is clean off whatever grime, dirt, cigarette tar, etc., that's on the surface. Use the usual suspects: Windex, mild soap & water, dilute ammonia, etc., whatever you like. However clean it gets, that's what you've got.
It's...you know...patina.:thmbsp: