daegaer pointed out this
useful site - monthly info, which is free and sends you reminders when your period is due. After three decades, I've never managed to keep track of my period, except in the vaguest of ways, usually through recognising the sudden drenching in pre-menstrual tension. I think this could be really useful.
eta:
cicer, I got your card at the weekend, thanks :)
son of eta: it had nothing to do with pre menstrual tension!