Why neediness kills, confidence rules, and ensuring success starts way, way before you walk in the room. When I was a copywriter casting commercials (both voice and on-camera), we'd see hundreds of people for a single role we had to cast. There were always some specs that an actor couldn't do anything about (gender, age, size, specific skill, etc.), but within those parameters, the one thing that made them far more attractive was something they had absolute control over: not needing the job. Of course, we all knew that anyone taking the time out of their day to read sides, get in the car, drive to an audition, park the car, and then park their butt in the waiting room wanted the job; the difference was that they weren't desperate for it.
Similarly, in production, they want to make sure actors are showing up to set prepared to do their part to keep the day's work running smoothly. If there's a meal that will be available, you're told that, along with what time they need you in makeup and/or wardrobe. If not, they ask you to show up "having had"—i.e., already put something in your stomach, if that's what you need to do, because there's not going to be a meal there.
I love this analogy for approaching work from a place of having taken care of your most basic hunger. Here are a few ways you can take care of yourself so you're not starving when you walk in the room.
Have someplace to go after the audition
You've heard this a thousand times by now—a few dozen of them just from me. But the reason this has ascended (or descended) into the Land of Cliché is because it's true.
When that audition is the high point of your day (or worse, the only point), you put an even greater amount of pressure on yourself to make it awesome. If you can something immediately after it that's cool to do, that's ideal, but even having something later on in the day (or evening) is fine. And the thing can be anything from seeing a movie or hanging out with friends to treating yourself to something cool like a massage, a hike, or whatever. ANYTHING. Do not vest that audition with your 100% happiness.
Have your nut covered before the audition
No, that was not a typo. (Although, you know, unless you're going to a very particular kind of audition, have those covered, too. Ahem.)
Walking into a job interview knowing that booking is the difference between solvency and living out of your car is not a confidence booster for most of us. Monkey work is not always a joy, but it really, really takes the pressure off of having to kill in every room. I kept my own Stupid Day Job—as the world's oldest gofer—for a full year after I was earning enough acting in commercials alone to support myself. And a full year after qualifying for insurance. (They were serious about keeping your address updated separately with Pension & Health—who knew?)
EXCEPTION: If you are one of those thrill-seeking types who successfully feed off of this kind of pressure, please ignore my advice and go for it. (Sadly, I have a similar thing going with writing and deadlines.) Everyone else? Keep your day job.
Get really good at one other thing
It can be a thing you make money at, like woodworking was to Harrison Ford; it can be a hobby, like photography was to Dennis Hopper (or is, I'm assuming, to Jeff Bridges). It can be something you have fun with sometimes and make money with at other times—my friend, Nick, built decks and furniture for money, and an actual canoe (that he paddled off the shores of Brooklyn!) for fun.
You can paint, throw pots, sew, design, dance, play an instrument (alone or in a band), garden, knit, write—whatever. Doesn't matter, as long as it's something that will engage you and it has nothing to do with acting.
Or nothing more than tangentially to do with it. In other words, singing and standup or improv are great, but they're still really close to acting-acting. Of course, it's great to do those things, too, but looking back, one of the things that kept me sane as an actor was being able to sit down at the computer and throw myself into graphic design. (I quit writing until the tail end of my acting career, mostly because some really bad experiences with screenwriting and sketch comedy sucked all the joy from it.)
Have one other, unselfish interest
Here's one I learned too late, being the most selfish person alive (according to an ex, anyway). But having done good works since, I wish I'd been doing them all along; nothing breeds confidence nor grants one a sense of perspective like being of service. Tons of good causes out there that can even utilize your creative skills. Personal plug: if you're a woman in Los Angeles who writes, check out WriteGirl, the nonprofit I raised money for; they do great mentoring work with high school girls from underserved areas of the city, they need all kinds of help, and I guarantee you'll be inspired.