Absolutely excellent idea that was emailed to me by fgt. It's two sheets of stickers printed with the most popular London Underground stations on them in the colour of the lines the station is on, asking people to wake you up at X station if you should fall asleep. As the site says "Whether it's a heavy night out, too much mulled wine at the Christmas bash or just a long day at the office, these handy stickers will help you get home safe and sound".
There's even some blank ones in the pack "for you to write your own stop on (Handy if you live in Osterley, whereever that is".
A commercial and funny idea of something I'd blogged some time ago with the same idea. "Another tip for not missing your stop and waking up at Heathrow (yes I've done this) is to set your mobile phone alarm to wake you up at roughly the time it takes you to get home, alternatively wear a big badge or a sign round your neck saying "Wake me up at XXXX station", although sadists would probably let you sleep on until the end of the line!"
Bargain at £5 plus p & p or you can buy them offline at Magma, American Retro, Aria and Best.