Originally Posted by Mr MCO
Another option is to join the BAA World Points (free membership).
http://www.baa.co.uk/
It offers 100 points upon joining. You can then redeem 50 points for a 50% off one way Heathrow Express, Gatwick Express or Stansted Express. 100 points for both ways.
And, if you refer someone, you get 250 points the first time they use their card. I just referred my wife.