This is a re-post of an older blog entry, but it goes along with the one I just stole from Jack...

YOUR/YOURE

Youre is always a contraction of you are. If youve written youre, try substituting you are. If it doesnt work, the word you want is your. Your writing will improve if youre careful about this.

If someone thanks you, write back youre welcome for you are welcome.

List of errors

THEYRE/THEIR/THERE

Many people are so spooked by apostrophes that a word like theyre seems to them as if it might mean almost anything. In fact, its always a contraction of they are. If youve written theyre, ask yourself whether you can substitute they are. If not, youve made a mistake. Their is a possessive pronoun like her or our They eat their hotdogs with sauerkraut. Everything else is there. There goes the ball, out of the park! See it? Right there! There arent very many home runs like that. Thier is a common misspelling, but you can avoid it by remembering that they and their begin with the same three letters. Another hint: there has here buried inside it to remind you it refers to place, while their has heir buried in it to remind you that it has to do with possession.

List of errors

There's a whole bunch more in the comments to the older blog post.