There's a few ways that I find work well, and they all work together too. The first thing is to embrace them, to accept that it's ok to be nervous and to know that everybody else feels it as well (even if they don't look like they do). Learn to see it as a sign that you care about your work and be proud of that. Secondly you need to change your perspective on auditions themselves. You need to learn to enjoy them and have fun. We should look forward to auditions, not dread doing them. If you're having fun instead of worrying about how you're doing then you'll definitely give a better performance.
Which brings me to the next, and probably the most important piece of advice. This is how you re-wire your brain so-to-speak. The audition IS the job. It is not the thing that you have to do, or some test you have to pass to get the job. Don't focus on trying to get the part. You already have it, for the time that you're in the audition room. For the time that you are in there the role is yours. That is your time to become that character. That is your time to show them who you've created from those words on the paper. That is your time to perform. Yours. So don't screw it up by not being there in the moment because you're nervous about whether you'll get the part or not. You have the opportunity to play that part RIGHT NOW. The audition IS the job, everything and anything that may come after is a bonus.
This is how I managed to get my audition nerves under control, so that now I can appreciate their presence as passion and actually enjoy the auditioning process. And it has honestly changed my world just by adjusting my mindset. I love my job! Oh and once you leave the audition room - forget about that job. You played the part, you did you work, and now it's time to look for the next opportunity. Get straight back to work and look forward, if you get a callback or the role then it will be a wonderful surprise! If you've been lucky enough to get offered two jobs while working hard like this then you'll be one of the few in demand actors who's able to choose which you'd like to do more. I really hope this helps any of you who struggle with nerves like I used to, and break-a-leg in your future auditions!