Behaviour therapists will tell you that the best way to correct an unwanted behaviour is to replace it with an incompatible behaviour. The example usually used is to prompt a child to clap when they're flapping their hands. It's impossible to both clap and handflap at the same time and clapping is more socially acceptable.
Coming up with a suitable incompatible behaviour taxes my creative powers but I had a good one yesterday.
We were at the dentist for Nathan and he kept reaching up to fidget with the tools. When I told him to keep his hands on his tummy, he started pushing them inside the waistband of his pants. So I needed to come up with a behaviour which would prevent both fidgeting with the dentist's tools and putting his hands in his pants.
My solution: hook his thumbs in his belt loops, like a cowboy.
It worked. As long as he had the thumbs in the loops, he couldn't stick his hands in his pants or reach the tools. And if a hand drifted up, I just reminded him to put the thumb back in the loop.