Hong Thai is currently selling Airport Express tickets at a 40% discount on its website:
http://ebuy.hongthai.com/tc/airexpress.html
You need to make your online purchase at least 24 hours in advance, as the "confirmation note" which you must print out and show at the Airport Express station ticket counter to get your ticket will be e-mailed to you 12-24 hours after you make your purchase on the Hong Thai website.