I've tried back-button focus a time or two, and never caught onto it. Part of the challenge is that my wife (and now my Dad) could possibly grab one of my cameras, and I know they'd be lost with back-button focus. I'm also afraid that I'll get a thumb cramp trying to hold that button in for a long time.

Instead, I've come up with a workaround: I leave my camera in AI-Servo, and set the AF-ON button to instead be an AF-OFF button. Half-press the shutter starts IS and AF like "usual", and holding the AF-ON button is effectively a focus lock. This works like a champ for me, and it's easy enough to teach to my wife.