My son tells me no quite often. The most important thing is to be patient and polite yet stand you ground. I am polite as I can possibly be.
Example: I asked my son to pick up his movies and put them back.
Me: Please pick up you movies and put them back
Me: Please do what mommy asked
Me: Your not being very nice to mommy right now. If you don't do what mommy asked you to then we will sit on the couch until you pick them up.
Me: Okay its time to sit on the couch.
I make him sit for a couple minutes then I ask him politely again. If he tells me no then we sit for longer. But most of the time I only have to ask him twice politely to pick them up.
I like to explain to my son why he has to sit on the couch. And sometimes i will help him. Only putting a couple things away then making him do the rest.
The more polite i am with him the better he is in all situations.