Parenting a tween or teen can make any grown-up feel like a two-year-old again – you know, where every other word out of your mouth is NO!
"No, you can't have a TV in your room." "No, you can't go to a midnight showing of Rocky Horror Picture Show." "No, you can't get a nose piercing." 
The next time your child asks you something that ordinarily would respond in a hearty, NO, stop and remember this axiom of parenting... "Say no when you have to and yes when you can."
For instance, let's say Jane wants to dye her hair a lovely shade of fuchsia. She's got the money to do it and "all my friends are doing it, too." Now, you know that more than likely she's going to hate it, and you will probably hate it, too.
But think about it... Is this an action that is going to cause her any kind of physical harm? Will it have a long-term impact on her mental health? Some of her peers will likely make fun of her, but she can always dye her hair back, or wait for it to grow out. Think of it as a learning experience.
Not every general wins every battle. The goal is to win the war... to turn out a basically good kid who has the tools to be healthy, happy and gainfully employed eventually. And every good general knows sometimes you have to give up some less important ground in order to achieve victory in the long run.
Sometimes it can even be fun to give in on something silly. Maybe it's having dessert before dinner, or letting them stay up late on a school night to watch a movie with you that you can talk about the next day.
Motivational psychologists say that having a sense of control over ones' life is the most important factor in mental well being. Think about that. Then think about how little control your adolescent has over their life. And remember the key word, "sense" of control. You know, and they should to, that the parent really has the control; but letting kids have more of a say, and being less rigid about saying, "okay," could really make a positive impact on both of you.
Another tactic that works well is saying, "maybe," and waiting it out. Kids are impulsive and change their minds as quickly as YouTube puts up new videos. The bigger the decision, the longer the waiting time between "maybe" and your final answer. This also gives you time to think about the best way to respond.
These kinds of situations are also an opportunity to teach your teen the art of presenting their argument. Ask your teen to make their request to you in writing, with pros and cons. Then calmly discuss them. One of you may find yourself swayed to the others' side. Perhaps there is a way to negotiate a compromise, or to use the request as a carrot to get them to do something they have been resisting: "If you get an ‘A' on your next chemistry test, I'll dye your hair myself."
Remember the mantra: "Say no when you have to and yes when you can." And don't let the opportunity to say, "Yes!" pass you by.