Do remove your faceplate completely from the receiver before cleaning it, for best results. [Reading between the lines, some here do but some don't). It's usually best to be fairly quick with the cleaners - a minute won't hurt, but DON'T soak them a long time. Apply cleaner, rinse, wipe or apply cleaner, wipe, rinse, but don't add "soak" into the sequence.
I've learned (from others experienced in the old-gear repair business and from my own experience) that --very strong cleaners aside-- usually what determines whether lettering comes off is the state that the lettering is in to begin with. Both how they were made and what they have been through since manufacture will determine how stable the letteris are. Best thing is to CAREFULLY test a small, least-conspicuous area of lettering first, to make sure it doesn't come off. If you scrub with something as harsh as a toothbrush, or even just a cotton ball, be especially careful around the lettering, not to rub (at least not hard). Again, avoid soaking too long, either, as it increases chances of the lettering (along with the protective coatings) coming off.
Use any of the cleaners suggested, avoiding the strongest and acidic ones, as suggested, and likewise finish it off with the automotive wax. In my experience, that last step is what really makes them "gleam" like new again. However, check the ingredients in the wax. The simpler the formula, the safer, although some protectants might help extend your gear's life.
Haven't tried the Flitz mag wheel polish that Rob recommends, but after his comments, I'll try that next. [Experiments usually don't hurt, and sometimes lead to improvements.] Probably already leaves behind a protective wax (or similar) layer, if it makes them look that new again with just one product used.