Learn to employ what I call "leadership speech" when giving instructions and communicating decisions. Use the fewest words possible, come straight to the point and do not give explanations. Explanations sound persuasive as opposed to authoritative. As such, they invite argument.
WRONG WAY: (The parent is scrunched down, hands on knees) "Honey, it would really help Mommy if you'd pick up the toys in the living room and put them away so my friend Susan and I can use that room to talk and have coffee in without a lot of distractions. Will you do that for Mommy, okay?"
To an instruction communicated in that wimpy fashion, a child is likely to say, "I was here first! Why do I have to move? And you never let me have anything to drink in here! No!" Mind you, the problem has been created by the parent. The child is only responding to the parent's non-authoritative presentation.
RIGHT WAY: (The parent is standing upright) "I need you to pick up these toys and move them to another room. I'll be back in a few minutes to see that it's done." (And then, walk away. Standing there will invite resistance.)
If, as you're walking away, the child asks "Why?" stop, turn around and say, "Because I said so. Any other questions?" And then leave the scene.
So, someone is bound to ask, what if the parent comes back in the room and the toys aren't picked up? Ah! Now a consequence is called for. But proper "leadership speech" will reduce the need for consequences by at least 50 percent within a month. First, stop repeating yourself. Give your child any instruction once, and once only. Second, pick the toys up yourself. Say nothing. Just pick them up. And then, immediately after dinner that evening, inform the child that he's going to bed. He is, after all, too tired to pick up his toys when told.
When it comes to consequences, be consistent, but do not be predictable. Be full of surprises!