On a teacher's thread it advised Saying it as an order once.
eg You have to go and feed the dog.
Annnnnnd thaaaaaat iiiissss itttttt.
Ensure you are in the situation where they can't pretend they didn't hear eg sitting at dining table. Then say nooooooottthhhhing else. Absolutley nothing. No 'did you hear me', no justifyig eg 'the dog needs fed he is starving' or 'the dog needs fed and I don't have time'.
Just say it then say nothing else, just wait. And wait. And wait.
It works with my DH and DS. Just wish I'd known this earlier.
I also found that if I wanted to know what, for example, DS had been doing I would say 'What were you doing at X's today' and waaaaaaaaiiiiiiiiiiit.
And evenutally maybe 5 mins later I would get the reply.
Strange beings these males.
Saw a similar thing on tv or somewhere where a gay guy was asked how he would get his partner to listen to him. He said when they were in the car together he would get him to pull over at the side of the road. Tell him to switch off the engine. Make him turn and look him in the face then say whatever it was. That way he knew he had heard.