Before you press "Submit"

Make sure the first letter of each sentence is capitalized, along with any titles, names and acronyms written out in your post.
Most web browsers have a built in Spell Check feature, use it! When you misspell a word it should automatically be underlined with red, you can right click it and figure out how the word is actually spelled.
You should generally avoid leetspeak. It's okay in moderation, but if you use it too much it could become a real eyesore.
You shouldn't mix up the words "Your" and "You're". Your is referring to a belonging of someone else:
While you're is a contraction for "You are".
It's the same for "There", "Their" and "They're".
There is defined as "in or at that place".
Their would be used when you are referring to an item of a third person.
They're is a contraction for "They are".
Avoid abuse of capitalization and punctuation.
Make sure to put an apostrophe ( ' ) when writing contractions.
You shouldn't mix up "Then" and "Than" either.
Then is used either as a time marker or with a sequence of events.
Unlike then, than is not related to time. Than is used in comparative statements.
Oh, and one more thing: Don't mix up "to" and "too".
The word too has two common meanings: (1) “also” or “besides” and (2) “excessively.” Sometimes people use it informally to mean (3) “very.”
The word to is used in all other cases—too many for me to describe in detail. Here are the definitions as listed in the American Heritage Dictionary.
Feel free to post more.