We adults like to have choices, so do the children. But we adult define the range of their choices by laying out artificial rules. For example, it is 10 PM now and my daughter is still on the Internet. Instead of shutting down the PC and telling her to go to bed now, I ask her, “Do you want to go to bed now or have 10 more minutes of computer?”  In case like this, I know she always chooses one of the less evils. You get what you want no matter what she chooses. Sometimes, I know she wants 20 minutes, still I ask her “How about 10 minutes?”  She bargains for 20 minutes and is happy to have it. I think people all feel good if they get what they ask.

Note this works until the child challenges the legitimacy of your rule.