The Heathrow Express is an eye-wateringly expensive way of getting from Heathrow to central London, but it is quick.
As such, anything which reduces the price generally proves very popular with the frequent traveller. This link will take you to a booking page which will automatically apply a Heathrow Express promo code (10% discount) to your ticket costs.
There are also alternative Heathrow Express promo codes which may work better for you, and indeed alternative travel methods to Heathrow. They are discussed here.