The one thing that can kill a good story to me is when there are too many grammar mistakes.  A lot of good ideas and stories have been lost to bad grammar and spelling mistakes.

On that note, if anyone would like an editor, I'd be happy to help.

Here are a few tips to tide you over:

you're = you are
your = something that belongs to you

they're = they are
there = a place (it has 'here' in it)
their = something that belongs to them (it has 'heir' in it)

then = sometime after; ex. "We went here then there."
than = for comparison; ex. "I like this more than that."

should of (or 'could of' or 'would of') = NOT A REAL PHRASE
should've = should have

it's = it is
its = something that belongs to it

who's = who is
whose = something that belongs to someone

effect = a noun; the result of a cause
affect = a verb; to cause a result

allowed = something that you allow; ex. "I'm allowed to drive."
aloud = something spoken; ex. "She read aloud."

Only recently realized this one is an issue.
where = a place
were = plural of was
we're = we are
As in: We're going to the lake where there were ducks last year.

I'm starting a list of commonly mistaken phrases:
1. for all intents and purposes (not intensive purposes, or other forms of mangling the phrase)

Hope this cleared up a few things for people.  Seriously though, I'd be happy to edit for anyone who needs it.