The Hong Thai 40% off discount is the best offer (that I know of) currently available for the Airport Express.
However, I also thought I would share other available offers that I'm aware of (if you aren't able to make use of the Hong Thai 40% off discount because of the need to purchase tickets online 24 hours in advance):
20% off single and return tickets
When purchased in person from Travel Expert travel agency (which has many branches in HK, including in Central MTR station and Kowloon and Tsing Yi Airport Express stations).
The Travel Expert branch in Central MTR station (shop number E17) sells a one-way ticket between HK and the airport for HKD80, and a return ticket between HK and the airport for HKD144. The Travel Expert branch in Kowloon station (shop number 53 located to the right hand side of the in-town check in area) sells a one-way ticket between Kowloon and the airport for HKD72, and a return ticket between Kowloon and the airport for HKD128. The Travel Expert branch in Tsing Yi station (shop number 302) sells a one-way ticket between Tsing Yi and the airport for HKD48, and a return ticket between Tsing Yi and the airport for HKD88. Airport Express tickets are also available from other Travel Expert branches (full list at
http://www.travelexpert.com.hk/en/Ab...ails.asp?Uid=6).
Group discount (2, 3 or 4 people travelling together)
Discount available on a single journey fare to/from the airport:
http://www.mtr.com.hk/en/customer/ti..._4persons.html
50% off an Airport Express single journey to the airport
If using an Octopus card, a 50% discount is available on a single journey fare *to* the airport if taking the taxi (taxi fare of HKD60 or more) to Kowloon or Tsing Yi station (but not Hong Kong station):
http://www.mtr.com.hk/en/customer/tickets/ael_taxi.html
Free connection on the MTR
http://www.mtr.com.hk/en/customer/se...nnections.html
btw would someone be able to help to create a wiki at the top of this thread (as suggested by the OP) where we can compile a list of all current Airport Express offers to make it easier to see all available deals? (I'm unable to create a wiki as I haven't had 90 posts yet)