Here's a tip for writing on polymer clay: a fine tipped CD writing pen doesn't smudge, and can stand up to being covered in varnish!
Some of my items, for example these Penguin Book Earrings (£8 in my Etsy shop!) require some detailed 'mock writing' (too small to attemt the real thing), and I used to try and do it with my tiniest paintbrush and black paint. But I recently discovered the usefulness of pens for writing on CDs - so hooray, I'm able to write with much more control! Try it out yourself :)