As a family counselor, one of the main things we encouraged parents to do to be effective was to set boundaries and maintain them. To say what you mean and mean what you say. Otherwise, your children will run right over you - begging, pleading, whining, and conjolling until you finally give in and let them have their way. Or, they will simply ignore you and do what they want knowing that there won't be any negative consequences.
Even having full knowledge of the importance of this, I struggle with it. It's especially difficult when you don't have a specific reason and so do give in eventually. However, I've found this gets me into deep parenting poopy. The next time I say "do this" or "no" my answer is not respected. It becomes a battle, of which there never should be. As parents, God has given us authority over our children and we are to teach them to respect that authority. Note: they need to be taught to respect our authority.
I have one particular child who is famous for pushing the envelop. This particular child will ask for the same thing a million times after I've given my answer. Even though I very seldom change my answer, and often state that I've already answered and am not going to change my mind, this particular child will keep on and keep on and keep on and keep on and keep on and keep on and keep on... (I think you get the idea.)
Lately, I've begun to stop this process. I will make it clear that I've answered the question, that is the final say on the matter and there will be negative consequences if the question is asked one more time. It has only taken a few times for the consequence to come with the words for me to get the process to stop mid stream.
"So harsh!" you may be thinking. But, I am reinforcing my God-given authority (which is ultimately important when they take the inevitable risk that puts them in major harms way - like walking out in the street, disappearing from our sight in a crowded area, just trying one joint, having unprotected sex - it is all connected) that puts safe boundaries around them until they are able to make greater and greater decisions on their own. If I don't establish that authority in the small things like respecting my answer on a requested activity, I won't have authority on the larger things as they get older.
So, as hard as it is to draw the line in the sand and maintain it, it is vital to effective, loving parenting. And if God has given us this authority, commanded us to let our words stand as they're said, then He will provide us with everything we need to do so.