Discount with National Express is available and there was something about showing your ID to the driver and you get a discount but this was a few years back.
There are 2 discounts on the coaches.
You can buy a discount card for £5 which lasts a year and you book on the internet or via CBS LHR as a young persons discount. You get a couple of quid off the full fare.
Then there is the airport commuter. That is a season ticket to any of the London BAA airports LHR LGW STN.
website is airport-commuter which will give you all the details.