Along with "Bell the Cat," "Jumping the Shark" is one of my favorite sayings. But many people don't know what it means. Just think of Cousin Oliver on the Brady Bunch or Scrappy Doo and you'll be on the right track. http://tvtropes.org/pmwiki/pmwiki.php/Main/JumpingTheShark defines it as follows: "Jumping The Shark is the moment when an established show changes in a significant manner in an attempt to stay fresh. Ironically, that moment makes the viewers realise that the show has finally run out of ideas. It has reached its peak, it will never be the same again, and from now on it's all downhill."
Here are a couple of sites (yeah, I know one is a wiki!) that you might find helpful.
For my symposium paper this coming January I plan to use video scenes from the appropriate "Happy Days" episodes and work forms of the word "jump" and "shark" in at least every 5 minutes. Should be fun!
For more, read the following: