I haven't read my copy for a year or two (must fish it out) but one thing that I have started using recently with my 7 year old ds that really works is this:
Instead of asking him to do something again and again and again, then getting cross when he doesn't do it, I ask him once, then if he doesn't do it I say
"Ds, tell me what I have asked you to do"
He repeats the instruction, then I say something like
"Great! Now I know you know I don't have to ask you again"
So far it has worked like a dream, no endless nagging/getting cross/ds getting upset