When waiting for your child welfare class to begin, do not mention that you will kill your little sister if she touches your clothes again...
Correct: I have a large group of friends.
Incorrect: I have a big posse.
No one sitting around you will hear you correctly.
When your friend starts saying "I am gonna see Orlando" Don't start saying "Get me an autograph!" You will just look stupid and it turns out that she was going to Florida.
If you meet someone whose child started reading at age two, and your first thought is "Wow, just like Hannibal Lecter," you either read too much or you're evil
When trying to convince someone to join you in a biology honors seminar, be aware of your phrasing. Correct: It will be fun; you could do your presentation on an interesting disease like leprosy! Incorrect: It'll be fun; you could get leprosy!
If you're washing your car, which is named Muff (due to the letters MUF being in the numberplate), and you are female, under no circumstances should you tell anyone you are washing your muff.
If you take almost exclusively after the pure german side of your family, remember that saying "I don't really care about the Jews" doesn't sound like you're complaining about the amount of reading for world religions class.