How to decide what to sing for an audition

Auditions are a daunting but essential aspect of the casting process. Ultimately you have a very short period of time to show your full capabilities so the pressure is truly on. You must be fully prepared to give yourself the best chance possible. Choosing what to sing for a specific audition is always hard, but begin by making sure you have a strong repertoire to choose from.
Firstly, decide what type of auditions you will most likely be attending. Will you be auditioning for musical theatre on the West End or as a pop singer for SyCo? What type of voice do you have? Are you more comfortable singing high notes (Soprano) in your mid range (mezzo) or the lower notes (alto)? What type of character are you most likely to be auditioning for or what type of personality will your pop style be? These are all questions you should spend time considering carefully before you chose your repertoire.
Now once you have answered all of these questions, start your own research. The best place to begin is with your own favourite songs. Maybe research the songs writer / composer to find more of their original works. Try to vary your repertoire, eventually you will want to create an extensive list but begin with only a couple, one classical and one contemporary to begin your focus. Allow yourself to learn these to the highest possible standard so you are ready for that last minute audition. Always be adding to your repertoire, once you can sing the songs in your list perfectly its time to seek out another, keep it constantly updated and fresh.
When choosing your repertoire look for songs that show off your range, if you are a soprano make sure you find songs that include your high legit notes, they will help you stand out from the crowd. If you market yourself as soprano, casting agents will want to hear this in your audition. On the other hand if you are an alto find songs that showcase the true power and strength of your chest voice. Look for songs that show off a range of techniques, you want to show how capable you are of commanding your voice. Be fully aware of the dynamics, tempo and characterisation in your choices and look for diversity to show the command you control over your voice. However, don't over complicate or stretch your voice, you want to show the best of your abilities so if you can't confidently reach that top E or sing that 'really fast bit' without proper pronunciation, just steer clear and work on it for your next song, don't take something to an audition unless you are able to sing it in your sleep!
Whenever you rehearse a new song make sure you practice with a vocal coach to ensure your technique is spot on. In some auditions you may be asked only to sing 16-32 bars of a song, your teacher can help you chose your strongest part and ensure the accompanist will not struggle with the music. This is a very important point, in my experience it is best to always have a back up with you, with every song in your repertoire you should own both the sheet music (for piano accompaniment) and a backing track, whether that be a recording of your singing teacher playing for you or the official backing CD. Always be prepared!
If choosing a Musical Theatre repertoire think about your casting type and characters you are likely to audition for and choose songs that are performed by the type of character you are likely to be cast as. This will be the same for whatever type of singing audition you choose to attend.
Once you have a repertoire of about 4 songs you will have a basic list to choose from with confidence and the hard bit is out of the way.
Before you attend your audition read and reread the specifications given to you, what do they want to hear? This is the best starting place. Next, check the role you have applied for, is it comedic? Ingénue? Older? Younger? Do they have an accent? Belong to a specific ethnic group? Now look through your repertoire and see which song best meets these specifications.
This will give you the most appropriate song for that audition.