I find that using Flitz Polish (non-abrasive and non-toxic)with a piece of old polo shirt material, and buffing with a clean piece of soft cloth, then using Obsidian Oil, as directed, to seal the stem, really makes 'em shine.
Doesn't it depend on why the stem is "dull" (oxidation or just grime) and what the stem is made from? Sometimes just a quick buff will work. Try This At Home How To Care For Vulcanite Stems
If you just need a quick brightening up, you can use a damp micromesh pad.
A couple of manufactures make stem polishing products. Obsidian oil and Brebbia stem polish are two. I've been using a product called, "denicare" and have been satisfied with it.
Lots of posts here in the pipe maintenance and repair forum too.
Perhaps this following German product would do the trick> Ebonite Polish 20g* Pfeifenmundstück entfernt verfärbungen und Gibt neuen Glänz' Ebay Germany, nevertheless,next time when I'm off to Americas(God only knows when this may happen)I shall buy me a tin of that all popular BarKeepers Friend