Introduce comic situations. Stretch the boundaries of what often happens in a realistic situation to achieve a comic effect. Revise a moderately embarrassing moment suffered by the story's main character while in private, for example. Make your main character, instead, undergo several ultra-embarrassing moments in a row, in a public place while with his girlfriend as an alternative.
Introduce the element of surprise. Pull the rug from under your main -- or supporting -- character's feet, for example, to turn a normal situation on its head. A normal diner conversation in the film "When Harry Met Sally," for example, introduced the comic element of surprise when Harry's friend Sally acted out an example of normally very private personal enjoyment in a very public manner.
Employ verbal humor. Create one character with an intellectual flaw, for example, by having him constantly speak in malapropisms. Malapropisms are real words mistakenly placed in a sentence -- unbeknownst to whoever is speaking. Make the character's first line "I granulated from college," for example. Add witticisms with another character. Follow Isaac Davis, Woody Allen's leading character in "Manhattan," who ironically comments on how easy it is to get by without a job: "I got enough (money) for a year. If I live like Mahatma Gandhi, I'm fine."