What worked for me - say what you mean, only what you mean, and mean it every single solitary time. Negotiate and give freedom on choices whenever possible, but when you have made a decision that something isn't up for a vote (such as safety, harming another child, etc.), don't confuse your child by letting them think they have any choice other than to do as told. Learning to live within boundaries, respect authority, and accept directions are vital, especially for our children who have minds that allow them to rationalize just about anything they want to do.
When I told my children no, it meant no. And if they defied me, I made sure the consequences were upsetting enough that they considered that the next time. You can't have kids defying you when what you've told them is to stay by your car because you're next to a highway and trying to get the baby out of the carseat. You just can't ever let them think that defying you is worth it or even a possibility under those circumstances.
By the same token, when it doesn't matter, don't make it matter. Don't put up power struggles where none need to exist. If your kid wants to wear a Darth Vader cape for weeks, let him. Who cares? If your kids wants to try peas and peanut butter and honey, let them. If they want to make a total mess of their room with Legos right after you spent an hour cleaning it up, does it really matter? If you're not having the Queen of England over, what's the big deal?
Our children do need to feel they have autonomy and freedom to begin choosing according to their own likes and interests. But they also need to know someone is in charge and looking out for them and keeping them safe. Not providing a balance of both is terribly confusing and frustrating for just about any child.
