I've been talking about training this since I got [access] to Morgan over a month ago. When she showed body language of wanting to interact with me, I decided to give it a go. We spent literally 2 minutes together training this.
I noticed when I lifted my right arm a certain way, she would raise her bum foot. I've noticed this since I brought her to my home, and knew I could cash in on it at some point to train the wave. I simply made sure to capture it with the clicker on the first time she offered, then shaped my cue around it and wham, bam, thank you ma'am, here it is!
A day later, Patty asked her to do it on the scale and she happily responded! I thought it might need more conditioning before she would do it for other people but nope, she's got it! The only other bird I've ever trained the wave to in two minutes is my galah, Bondi!
For more info on the actual TRAINING details that go into this trick, click here.