My stamps are tools I use to make pretty things. I love them, but I don't coddle my stamps. I use them and to me, a stained stamp is a loved stamp. I have purchased used stamps that came severely stained. The image was perfect, but the stamps were really almost thick with the old ink. I wanted to clean the stamps without hurting them. I tried all the stand by stuff - the SU! cleaner, the TAC cleaner, even Stazon cleaner. None of them worked to get much of the really old gunk off. I had read somewhere about conditioning stamps after using harsh cleaners on them to keep them supple. I bought glycerin online, but you can get it at Walmart in the health and beauty section.
I took a foam plate, coated it with a thin layer of glycerin. I added a few drops of Dawn dish detergent (probably any detergent with degreaser in it would have been fine). I mixed it around with my finger and laid a few of my stamps in them face down. I left them overnight, and the next morning, I was amazed. When I lifted the stamps out of the ooze, they were clean, but even more interesting to me was the ink that was left in the goo. In some cases, there was almost a perfect image floating in the mix, suspended in the liquid, and no longer sticking to the stamp. Very cool. Not all of them did that. I think the ones that were the gunkiest - you know, I should get on the crew that writes the dictionary or something - they were the stamps that left images behind and not just traces of ink.
Anyway, since I know a lot of people aren't like me, and they actually want all their stamps to be as unstained as possible, I thought I'd share this secret formula I figured out almost a year ago. My stamps also were very soft and supple afterward, so that was a bonus!