1. They are used to denote a missing letter or letters
  2. They are used to denote possession …
    however, if there are two or more things the apostrophe comes after the ‘s’
  3. Apostrophes are NEVER ever used to denote plurals!

You can read the full version over at The Apostrophe Protection Society, which is where I first saw it.

Update: here is a poster which illustrates it very well.