What sorts of things are you saying no to?
I want a cookie!
You may have strawberries or crackers, what's your choice?
I want a cookie!
Maybe we'll have one later, after dinner.
I want a cookie!
We already had one cookie. One cookie a day, that's the rule.
I want to go to the playground!
We will go to the playground after lunch.
Play with me!
I will play with you as soon as I finish folding the laundry. Would you like to help?
(yelling gibberish at baby sister)
You may talk to her in a normal voice, or you may play in your room, but it's not okay to yell in her ear.
I want one more story before bed!
We already read two. We'll read more tomorrow. You may take a book to bed and look at it.
etc. etc. etc.
Mostly I just try to offer alternative choices (emphasis on what ds CAN do, not what he can't do), or defer until later. Good luck!