Consider this: casual writing arises from casual speaking. We naturally use a lot of contractions in our speech, most often without thinking about them. (A contraction is the mashing together of a noun and verb am/is/are.) You’re (you are) and they’re (they are) are ubiquitous in speech, and they also lead to some of the most common—and damaging—writing mistakes.
Want to shoot yourself in the foot? Use your in your copy when you actually mean you are or you’re. And don’t confuse they’re with their or there. It’s a common mistake because they sound the same, but there is no quicker way to torpedo your communicative efforts.
How’s this for an example:
Your an excellent customer. Were doing our best to send your order as soon as we can. You’re widgets are almost ready. Their coming by Priority Mail.
Your and their are possessive pronouns, to be used only with a subsequent noun, e.g., your shoes, your money, their jobs, their self-respect, indicating that the noun belongs to or is associated with the pronoun.
When you catch yourself doing this and think, “Oops! What a dummy!” that might be what your boss or your customer would have thought, too, had you sent that communication through.
A good way to avoid this altogether is to avoid using contractions in your writing, or at least using them much less. Be aware that writing is not the same form of communication as speaking. It requires more time and more stringent adherence to rules to look professional and be effective. Spell out you are, they are, we are, etc., at least until you are sure of the difference.
It will make your communications look more professional and eliminate the chance of saying something like: There running a good business over their with they’re new CEO.