If you happen to be writing about "high school" or something of that sort, don't forget the stereotypes! Readers -love- when your story is stuffed with the following:
Preps - Make sure that these characters are perfect and blonde. They never wear anything but Abercrombie and Fitch clothing, and females wear a lot of lip gloss.
Emos/Goths/Generally misunderstood kids - These characters should always be found lurking in inexplicably dark corners of hallways, in alleys, ect. They moan and mope around, searching for their purpose in life. And they MUST have a tortured past that keeps coming up for no reason, or your story will be a complete failure. Remember to make them main characters.
The totally hot and popular guy/girl - Who cares if he/she has no personality? Certainly not your readers. Remember to hint that this character has a deeper side, but never bother to show any of it. He/she can never be a main character, just something to be observed from a distance.
The tragically misunderstood popular guy/girl - This character may be extremely popular, but do to a tragic past, they can't be happy. They spend their days searching for who they really are. Remember to bring up that fabulous tragic past over and over again. This is usually a main character.
Also, this has been stated many times before, but NEVER bother to use proper grammar or spell out your words. No one cares if you use "ur" or "k". All that matters is that your story gets written out as quickly as possible and looks hurried and thrown together. It sooo makes you look like a tortured artist.