Well be sure you notice the following

1. Taxi Connection for HKD 60+ gives you 50% off for ex-KLN / ex-Tsing Yi Airport Express Ride

Ask for the receipt from the cab driver, bring the Octopus to the service counter with the receipt. the AEX ride will be 50% off
http://www.mtr.com.hk/eng/whatsnew/promotions_taxi.html

2. Play the MTR "Butterfly" app and collect the bonus for 50% off AEX ride

Never tried so dunno the details
http://www.mtr.com.hk/eng/whatsnew/aelibutterfly.html

3. AEX Frequent Rider Bonus (applicable even you take the taxi connection discount)

https://www.mtr.com.hk/eng/ael/index.html#p1
4 ride = 200 AsiaMiles
7 ride = 500 AsiaMiles