I'm sure you're going to appreciate me throwing this in here, but I never understood this properly before (sorry, SealPoet, but I din't get your explanation)
TIP #1:
There is an easy way to remember the metre.
Recite out loud the first line of the old Christmas classic, 'Twas the night be-fore Christ-mas and all through the house. In this line you can hear the compelling beat: da da DUM. This unit of the beat is called a metrical foot but, unlike the Christmas poem, a limerick contains three metrical feet in lines one, two and five, and two metrical feet in lines three and four. Hence:
da da DUM da da DUM da da DUM
da da DUM da da DUM da da DUM
da da DUM da da DUM
da da DUM da da DUM
da da DUM da da DUM da da DUM
http://female-orgasms.com/limericks/