Micro stories and flash fiction

Quoting wikipedia, flash fiction is fictional work of extreme brevity, including the Six-Word Story,140-character stories, also known as twitterature,the dribble (50 words),the drabble (100 words),and sudden fiction (750 words).
Here are some examples:

Six- Word Story
"For sale: baby shoes, never worn." is the entirety of what has been described as a six-word novel, making it an extreme example of what is called flash fiction or sudden fiction.

Twitterature: Harry Potter
from http://www.twitterature.us/us/ex.htm
Hello everyone from under the stairs! Aunt and Uncle threw me under here again. Gosh, life is so hard.
OMG I'm a WIZARD! And my parents are DEAD WIZARDS! Off to magic boarding school.
OMG the year's over. Time goes fast when you're having fun. Goes slow if you have to read seven books with lots of adverbs.

100 word story
from http://www.100wordstory.org/6273/calling-mom-home/
Calling Mom Home
I call my mom once a year, on the day she died. Five times I have pulled up “Mom Home” in my contacts. Five times I have pressed the phone icon. Five times she has never picked up.
I haven’t given her much time. I tap the red disconnect button as soon as I hear the first ring. “Call ended” flashes across the screen, and she is gone.
I worry someday someone will answer. Someone who is not my mom. Someone whose number this is now. That someone might call back, ask who this is anyway.
I would tell them.
Elizabeth Boquet is a Louisiana writer living in Connecticut. She writes in the space between New Orleans and New Haven. 

How to write flash fiction?
Use a small idea.
Start in the middle of the story, at the beginning of the conflict
Take out all unnecessary words.
Focus on one main conflict.
You don’t need more than one or two charachters.