Apostrophes redux, short form

The only times you should use an apostrophe are when:

• You are contracting two words together, such as when "they are" becomes "they're" or "she is" becomes "she's." The apostrophe goes in for the missing letters.
  
• You are describing something that belongs to or is part of someone or something, such as in "David's dog" or "a hard day's night."

It's simple. Really!

Note: The long, somewhat bitchy form is here.