He's finally done it but..............

[Cayley]

We were told to slow down the word and really pronounce it so they understand the sounds of the word and the association with the action. So imagine the dog is just about to get into the down position, when he's going into the down position you say 'dowwwwnnn' instead of 'down', gradually you'll be able to shorten the word to down and he should do it  .
   

[bluegirl]

I've found it, it's called British bird dog basics by Derry Argue.