After a while cloth mousemats can get a bit manky. I stick mine in the washing machine, it still doesn't fully return it to that new feeling but I've finally discovered the solution!
I bought some of that for squeaky door hinges, but I know PTFE is the same stuff they make gaming mouse feet from. So I sprayed my mousemat with it - it worked like a charm. It feels like new again and doesn't seem to have affected the mouse tracking at all.
Hopefully it helps someone...