What are the main rules to writing a limerick?

Answer 1

It's actually a tad more complex than many poem structures.
Here are the basics:

1. 5 lines.
2. Rhyme structure is AABBA (lines 1,2 and 5 rhyme, and 3 & 4 rhyme with each other. Their "beat" is di di dum di di dum di di dum
3. Lines 1, 2, and 5 generally have seven to ten syllables, while lines 3 and 4 have only five to seven syllables. Their beat is di di dum di di di dum...
4. Ideally, it should be about a humorous or clever story/observation/statement, with the last line carrying the real "zinger" that makes it truly funny.
   

Here's one that I wrote for a site called OEDLIF (The Omnificent English Dictionary In Limerick Form) where they are trying to define the entire English language using limericks (Link below). The limerick is meant to define the term "breakfast bar":

There's a breakfast bar in our new home -
My wife had it detailed in chrome.
A bar? To make trouble,
I ordered a double!
(She gave me espresso with foam.)

Answer 2

A limerick follows a five-line rhyme scheme (AABBA). The first, second, and fifth lines should have 8-9 syllables and the third and fourth lines should have 5-6 syllables. Limericks often have a humorous or whimsical tone.