RULE 1:  Use familiarity, word associations and common assumptions.   So take your subject and list phrases, synonyms, stereotypes, sayings, people, places and things connected with it. .
RULE 2:  Exaggerate things to the extreme.  
RULE 3:  Be brief.  
RULE 4: Twist the joke. Give out a real fact in the set up, and make up a crazy (exaggerated) fact in the punch line.  
RULE 5: Speaking of punch lines, make sure the last word, or pretty darn close to the last word, is the zinger punch line.