I just learned how to do AH while learning the song "Laid to Rest" by lamb of god. I'm still practicing but I can do it without focusing on my hand too much.
The way I do it is to literally hold the pick right at the tip, and play with both your thumb AND pick. Pick the string and then immediately touch the string with your thumb, but just a brush, don't mute the string.
I've also found it helpful to play with the pick at an angle. Don't hold it straight down, but twist it so its facing the bridge, and play with your thumb at that angle. Of course its impossible to play a SONG like this, but when doing the pinch harmonic you will notice your thumb brush down towards the bridge.
Also you can hear it best when its HEAVY distortion. I've done a pinch harmonic with clean and its pretty hard to tell. Distortion makes it very obvious, and it sounds awesome once u learn it.