It seems that the most important factor in your being able to influence your teenager is the strength of your relationship with him or her. When there is a good parent/child relationship, a teen is more apt to have a healthy view of life and will be easier to motivate than others.
If a parent who has a superficial relationship with his or her teenager tries to motivate the teen toward some type of academic or life skills goal, chances are slim that the methods will be effective. Simply lecturing a kid, or telling him or her what to do, will actually undermine any effort that you have made to spur your teenager on toward loftier goals. Another way to undermine your efforts is to condescend to your teenager.
Teens need to be treated as whole people, not just as students, children or delinquents. They also need to be shown how to be successful. If you can help your teenager to develop a positive attitude about life in general, they’re more likely to develop a higher degree of motivation. Stress to them the value of their education. You also should spend some time asking your teen questions so that you can better understand their motivation or lack thereof. Ask your son or daughter what their goals are. Ask how you can help and then make yourself available when they come to you. If you put them off time and time again, chances are that they’ll stop asking for help altogether.
As you set goals together, also discuss consequences for not meeting those goals, i.e. not completing homework, not doing physical exercises, etc. Teens need all the help that they can get. If you don’t offer it to them, they’ll seek it out elsewhere. In order to ensure that the help they get is help that will motivate them toward higher goals, it’s best that you, as a parent, are the source of most of their help and encouragement.