Thanks for the request, @ChrisK! I loved @miggles36's take on this. The whole enterprise is very personal and subjective. That said, here are my two (hundred) cents, mostly having to do with television.
  1. DO prepare.
    Read the script. If you did not receive a full script, request one. If it is not available, watch episodes of the TV show or dig up the director's reel. Go to IMDb & see what else the writer has written. Know your lines, motivations, obstacles, tactics and inner life. Build a history. Know the world & the genre. Make interesting choices, but make sure they're text-based. Don't invent shit out of thin air when the writer has given you a clear road map. Rehearse the scene so it's in your bones.
  2. DON'T stress out about feeling unprepared.
    Had to work until 2am? Got the material the morning of the audition? Didn't have time to go too deep with the prep? First of all, calm down. Secondly -- and listen closely -- do the best you can with the time you have. Use your script if you're not memorized. Everybody knows it's an audition. If you only have time do to one thing, develop a point of view. That is the single most important part of your preparation. The rest of acting is just listening.
  3. DO look the part.
    Use common sense. Don't wear a smokey eye to the audition where you're supposed to be a farmer. Don't wear sweatpants to the audition where you're supposed to be a District Attorney. Looking the part helps you get in the mood. It also helps the imaginations of the people watching.
  4. DON'T wear a costume.
    Ever. Do not ever where a costume. Also don't take the "dress the part" thing too far. You're a blank slate. Let the viewer project onto you. If you are too costumey you will look ridiculous, cartoonish and way too specific. The work must be specific but the look must allow room for imagination.
  5. DO bring a change of shoes (ladies, this means you).
    I know. It's sexist and unfair. Men shower and maybe put some pomade in their hair and walk out the door. Women have to primp to prepare. It's a major time suck. ANYWAY, you do not want to find yourself traipsing across the Universal backlot in 90 degree heat while wearing stilettos. Trust me. I've done it. And those "walk of shame" vibes plus aching feet do nothing for your pre-game headspace. If you're a model and can wear flats, bless you, you gorgeous thing, you. Love Always, 5'2"
  6. DO improvise as your character.
    Mostly I mean in the car on the way there. Someone cuts you off in traffic? How would your character react? Can't find parking? Small talk with the security guard? Need to ask where the sign in sheet is? Do it as your character (within reason). This obliterates nerves, takes you and your silly little ego out of the equation and gets you in the fun headspace of doing what you came here to do!
  7. DO practice healthy self-talk.
    This may be too hippie dippy for some of you, but for those interested: talk to yourself. At my most nervous/vulnerable/afraid I will literally stare at myself in the rear view mirror before exiting the car and say, "I love you. I approve of you. You are enough." I will repeat this until I sort of believe it, and then I'll get out of the car and jump feet first into the lion's den.
  8. DO medicate if you need to.
    Concert musicians take beta blockers. One of my best friends did a whiskey shot before she tested for her series (that she booked and went on to star in - ahem, thanks whiskey). @mindy said in Why Not Me that a tequila shot before an important meeting is totally acceptable. And she is someone whose life/methods/humor/choices I admire tremendously. Do what you gotta do. And bring gum or mouthwash if "what you gotta do" involves alcohol.
  9. DON'T medicate if you don't need to.
    I took beta blockers a couple of times and I thought my heart was going to stop inside my chest. That provided a whole other layer of worry, which did nothing to help me (though I will say my network test was very chill). Don't drink or pop pills unless you're sure that what you're doing is necessary and helpful. And don't test drive this stuff on an important day. Try it out beforehand.
  10. DO arrive early.
    You've got enough to worry about. If you're late, you will drive recklessly, feel rushed, be short with security guards and forget to catch your breath before you start your audition. Prevent the preventable. Arrive early.
  11. DO be mindful of your waiting room psychology.
    Be nice to everyone. Do not be a pleaser or seek anyone's approval, because fuck that. But be nice. This is your community. If you see people you know, great! Talk to them. It will get your mind off things and get you listening & responding. If the whole thing is too much, carve out your own space. This is what I often do, because I'm an introvert and waiting room dynamics freak me out. Move to a private area or go outside. Do whatever you need to do to feel calm and ready.
  12. DON'T ignore your nerves.
    Nervousness is energy without breath. Breathe into that shit. Own it. I find that if you name something, that thing loses most of its power over you. "I'm nervous. My heart is beating fast. My palms are sweaty." Okay, cool. You're probably not alone. You're alive. This is part of the whole thing. Now that you've made friends with your nerves, you can move on and get to work.
  13. DON'T view others as competition.
    Oh, honey. These people aren't your competition. The celebrity who is getting the offer while you sit cluelessly in the waiting room isn't even your competition. The Aussie they're going to fly in to play a girl from your actual American hometown isn't either. These people are you. They are in the same boat. Treat them with compassion. Your only business here is to do YOUR very best. I'm a highly competitive person, so you have to believe me when I tell you this.
  14. DO use your body as an instrument.
    You are not a talking head. Use your body. I mean, don't go nuts. Be specific and intentional. But don't be one of those floating cerebral automatons spitting out lines. Speak from your diaphragm. Make sure you are breathing and that your throat is open. While waiting, don't cross your arms and legs. That closes you right off! Stand up, open your heart, stretch out. You may feel silly (if so, hide in the bathroom or in a hallway like I do) but this is going to warm you up in the best way.
  15. DO come in with a gift rather than a need.
    They can smell desperation from a mile away. I know you need to pay your rent, meet your health insurance minimum, give your mom some fodder so she stops asking questions & live out your dreams. Forget about all of that. Go in thinking, "What can I bring to this? What can I contribute here?" Whether a smile to someone in the waiting room, a moment of connection with a casting director or a laugh with a producer, it will feel like a win. You have control over what you give, not over what you get.
  16. DON'T ask ridiculous questions.
    Can I use a chair? Where does this shoot? Does this character have a boyfriend? What are they trying to do in the scene? Is it supposed to be funny? NO, NO, NO, NO, NO.
  17. DO ask smart questions.
    They will say, "Do you have any questions?" If you have an actual, good question, ask it then. If not, please avoid staring at the ceiling and wasting everyone's time. A decisive "no" will suffice here. Unless it's a 3rd callback and you're engaged in a conversation with these people, you're not here to talk. You're here to act.
  18. DON'T make producers characters in your scene.
    It freaks them out when you look them in the eyes. Like if you were watching TV and the character broke the fourth wall and looked at you, that might pull you out of the story, right? Let them watch their story play out in peace.
  19. DO be yourself.
    Nerd alert: I can't tell you how many times I've practiced saying "Hi" and "I'm good! How are you?" in the car on the way to auditions. Don't do this. It's sad, embarrassing, unnecessary and will make you feel crazy. TV moves fast and you'll end up spending a lot of time with these people if it all goes well. They will figure out who you are. So get a head start and be yourself right now.
  20. DO respect the lines.
    So someone bought this script, and you're auditioning to be a part of the show. That means the words already have a job. You, on the other hand, do not. Respect.
  21. DON'T be shackled by the lines.
    It's okay to go off book, improvise, paraphrase, etc. if you don't remember the exact lines. Anyone can recite lines. They're looking for a person. An essence. A character. A vibe. Exception = Sorkin. Do not change a single word if you are auditioning for Sorkin. I also find this to be true in comedy, which is written almost like music. They've chosen the words quite intentionally, so it would be really good if you could say them as they're written. As long as doing so doesn't make you a robot.
  22. DON'T apologize or ask permission.
    "I'm going to stand for the second scene." See? Wasn't that easy? If they really disagree with the choice, they'll tell you. And when you finish, never say you're sorry. Whatever the hell just happened? Own it. (Exceptions = spilling coffee on a camera, vomiting on someone's shoes, bodily injury, etc. Then you may apologize.) You can even ask to try it again if you want (and they might say no). But sorry chips away at that whole mutual-collaboration-&-cool-confidence thing! No sorry. Be a boss.
  23. DO take your time.
    Slow down. Everyone around you may be rushing, but slow it down and take up some space.
  24. DO know your eye lines.
    Most auditions are on camera. Establish the space and the other characters in the scene with your eye lines. Regardless of your choices here, make sure the camera can see both your eyes at all times. We can't connect with you if we're staring at the back of your head.
  25. DO make the reader the star of the scene.
    Counterintuitive, right? This is actually not about you! It's about your interest in and effect on the other person! Make that reader the most interesting person anyone has ever seen. Acting is a team sport. What you do when you're listening is probably more interesting than any line you say.
  26. DO have a plan and be ready to throw it away entirely.
    Be invested. Come in with a plan you're excited about. Then hopefully they will redirect you and you can throw your plan out the window and be super adaptable. Easy to make a choice. Hard to be facile.
  27. DO be believable.
    Great choices. Well executed. Only we don't want to watch an actor executing choices. We want to watch a believable person living through an experience.
  28. DON'T be an asshole.
    This should be self-explanatory. Sometimes people will say mean things to you. They will make you wait 40 minutes in a crowded waiting room. A phone will ring during your audition. Someone will be texting during your scene. They'll give you direction you disagree with. You do not know those people's lives or motivations. Check your ego at the door. Don't be an asshole.
  29. DO be kind.
    Kindness is king. In auditions, in work, in life. There is absolutely no reason not to be kind.
  30. DO have fun.
    It is only fun to watch people having fun. Even in a sad scene you should, on some level, be having fun. This is the single most important thing I can tell you. If you are not having fun, we're all fucked.
  31. DO let it go immediately.
    I usually give myself the car-ride home to a) celebrate an awesome audition, b) cry over a bad audition, c) second guess everything that happened, d) redo the audition 8000 times in my head, e) dream about season 7 of my new show, f) catastrophize, g) mourn the fact that I will never work again, h) wait with tween-level intensity for the phone to ring. And then: NO MORE. Move on. It's over. Get busy doing something else. Drop it!
  32. DO be ready for either outcome.
    The Samurai does not fear death. There are 2 ways this can go: you'll get it or you won't. Make peace with either, and then neither will come as a surprise. Life is a series of victories and defeats. Don't be fragile about it. Take it all in stride.
  33. DON'T feel obligated to say you like this.
    Those people who say they love auditioning? They're a) aliens, b) lying, c) delusional, d) on drugs. Sure, there are those life-affirming audition experiences that make you feel giddy afterwards. But normally it's a bizarre pressure cooker riddled with judgment and rejection. Don't feel like you have to be like, "I love auditioning!" A healthy dislike of the process is normal. But if you want to do this, auditioning is going to be part of it. Period. Make your own kind of peace with that.
  34. DO repeat after me:
    I am not an actor. I act. I am not my job. I am a complicated human with an infinite spirit. I am not my résumé. I am a person with passions, friends, family, experiences and a rich future. These people's opinions don't matter. My opinion matters. The opinions of my loved ones matter. Do I like who I am? Am I living according to my values? Am I contributing something? Am I having fun? What else is going on in the world today? Good luck and Godspeed!