I would modify MBSLyne's approach by phrasing it, WHEN you complete "such and such", THEN you will be allowed or entitled to do "such and such."

    I take out the negatives unless it is a situation I have failed to address beforehand such as something they were supposed to do and didn't. Then you say, "If you don't do...then you can't..." Whatever it is you say they can't do, it must be something they want at that moment. If your reward for desired behavior is a thing they don't want at that moment, it won't mean enough for them to do what you ask immediately!

   With the above example regarding DSS with my son, he scrambled to sign up because he knows I will follow through on my threat! I had told him originally, he couldn't have his truck on campus UNTIL he got a job. What did he do? He got a job immediately and I gave him his truck.

    As a result although he is no genius and I don't know whether he will be a success in adulthood, he has never had a wreck, no tickets, no arrests, no drug use, no alcohol, no cigarettes, and no pregnant girlfriends. He joined his own church of his choice... and he is a Junior in college which amazes everyone who knows him. Knock on wood. (We won't mention the machete incident,   will we Dr. Peterson)!