I used to very carefully paint the top surface of the numbers, but they would wear off very quickly, now I just use tippex dots just to the left of the raised numbers and they have survived 5,000km worth of shifting as they are out of harms way.
I have gone for 2 dots on gear 11, one dot for 7 and a final dot for gear 1. I have found that I don't use these at all when I'm riding, as I am so used to the gears now, that I can guess which gear I'm in anyway.
Where I do find them useful is for picking a gear to start off in.