Do leave it all at the door. This is a tough one, because of course you want the part. But the moment you leave the casting office or submit your tape, forget about the audition. You'll cause yourself unnecessary stress fretting over whether or not you gave a good performance. Let it go and move on. You might be pleasantly surprised a few days later if you get a callback or even better, get booked. But if that call doesn't come, it won't matter because you will have already moved on to the next thing.
Do prepare different versions of takes. This is especially important if the sides are short, like for a commercial.
Don't try to impress. It sounds counter intuitive, but the moment you walk into the room and try to impress those on the other side of the camera, you run the risk of coming across as disingenuous. Casting directors can tell if you are trying too hard.
Don't play the role a certain way because it's the way you "think" they want it to be played. Casting directors want to see YOU and what uniqueness you bring to the role. They don't want to see you come in and try to read their minds, acting in a way that feels inauthentic. You might be the perfect person for the part, but they'll never know it if you are focused only on delivering the copy in a way you've decided they want to hear it.