removing tea stains from ceramic mugs

[frecklestars.livejournal.com]

My last (idiot) flatmate used to leave tea sitting for days in my white-inside mugs.  Now one of them is permanently stained: I love the mug (it reminds me of my great grandmother), but I don't love the nasty stains inside.  Any ideas on how to get it clean?  I'd strongly prefer not to use bleach or other really nasty chemicals, so less harsh solutions are a plus.  And I want to continue to drink out of it after I (hopefully) get the stains out.

Thanks in advance.