Girls and Boys Town has some nice advice for putting your child in time out. It's served as a nice reminder to me. Here's the quick and dirty (there's an expanded version on their web site):

1. Choose time-out areas.
2. Explain time-out.
3. Use time-out every time the problem behaviors occur.
4. Be specific and brief when you explain why your child must go to time-out.
5. Do not talk to or look at your child during time-out.
6. If your child gets up from the chair, return him or her to the chair with no talking.
7. Your child must be calm and quiet to leave time-out once time is up.
8. Your child must answer yes politely when you ask, “Would you like to get up?”
9. If you wanted your child to follow an instruction, give him or her another chance after time-out is over.  And, in general, deliver a few other easy-to-follow commands so your child clearly learns who is in charge and who is not.
10. Catch them being good.