To Tip or Not to Tip... That is the Question.

I just came across a great article on tipping. People often don't know when to tip, or how much to tip. Yahoo News featured an article with advice on the subject from Anna Post of the Emily Post Institute.

When in doubt, always remember that it is marvelously well-mannered of you to show your appreciation for someone else's consideration and hard work. What do you think? What are more instances where one should tip and what is your rule of thumb? I would like to hear from people on how they handle the tipping situation when they encounter poor service.