A conversation took place in another thread that talked about tags, when I find it I'll link it to this thread so we can see it under this subject. But in the meantime I've used two different methods to tag my skeins when I want to track what I did with mordants and assists.
1. High tech materials - The little pudding jars that you get at the grocery hold up well to heat. I cut small disks and put a number on them with magic marker, punch a small hole and attache that to the skein. Of course you need to test how your marker and the plastic tags hold up in the dyepot heat.
2. Low tech materials - When I am prepping my skeins I identify them with extra ties that have 1, 2, 3 or 4 knots in them. If I have more than 4 skeins I just to to two ties that have a 1 and a 4 on them. I've found if I go longer with 5 knots the marker tie gets tangled with the skein threads.
Both of these are easy to find when you have a 5 gallon tub of wet skeins and you need to pick out a particular skein.