I'll endeavor to make this brief. Per the suggestion of one of my readers, I thought it might be a good idea to address the whole “you're“/“your” issue. Since he brought it up, I have to say that I have actually seen this mistake fairly commonly in texts that I have edited, so it's probably good to mention it. And while I'm at it, I'll take out the “its“/“it's“ and “they're”/”their” issues as well, since they're the same issues, more or less. These are things that are sure to upset your editors if you overlook them.
“You're” is a contraction of “you are.” “It's” is a contraction of “it is” or “it has.” “They're” is a contraction of “they are.” The apostrophe (') indicates that something's missing. In the case of “you're” and “they're,” we're cutting out the “a” in “are.” In the case of “it's,” we're either taking out the “i” in “is” or the “ha” in “has.” In all cases, you can see that we are squishing two words together. This is just a convenient way for us to represent how many English speakers speak because we tend to slur things together to speak in a more fluid and fast manner. French is much worse about this sort of thing, but most of the languages I've studied have their share of squished words. :)
On the other hand, “your,” “their,” and “its” are all possessives. Now if you go read that page, your head might hurt afterwards, but you should have a fairly decent grip on the idea of what a possessive is if you don't already. The point here is not a lesson about possessives but rather to illustrate that these three words are possessives, and you need to ensure you use them as such.
However, I don't think most people generally accidentally use “they're” or “it's” when they mean to indicate the possessive. Usually the error is the opposite, i.e., they use “their” and “its” to indicate their related contractions. So, for instance, when they mean to say “they are,” they might use “their” or use “its” when they really mean “it is.”
So if you have had this problem in the past, be sure to be extra careful when you go to use the contracted forms of those verbs. Remember, you are squishing two words (to make a contracted verb), so you need the apostrophe to indicate that's what you're up to, to tell us that you've taken something out. If you don't remember this, your editors will beg Zeus to strike you down in a flash of lightning.