1. Is this song in the style of the show?
What does this mean? If I am auditioning for Natalie in Next To Normal, I should not walk into auditions with Think Of Me from Phantom of The Opera. Next To Normal is a comtemporary musical, meaning I should sing a song from another contemporary musical. Or if I wanted to be even more "in the style of the show" I could sing a song from another musical with music by Tom Kitt.
2. Does this song fit the character I'm audtioning for?
If there is a specific role you want in a show, you should choose a song that reminds you of the character you're auditioning for. Now, that does not mean you should audition for Diana in Next To Normal with Forget About The Boy from Thoroughly Modern Millie. That would be horribly ironic.... and not in a good way.
3. Would I be able to sing the entire song if they asked me to?
Being able to sing your audition cut is extremely important, but being able to sing the entire song upon request is just as important. If a director wants to hear you sing the rest of the song, It usually means they are interested in you. Saying "No" because you don't know the rest of the song will most likley make them lose interest in you very fast.
4. Is this song appropriate for someone my age to sing?
Of course, every rule can be broken within reason, but generally, if you are 12 years old, you should not be singing The Winner Takes It All from Mamma Mia (But really, should anyone really sing that?)
5. How likely is it that other people will be singing this song?
If you are planning on singing a song that usually shows up on the "Overdone Audition Song" lists, the chances are probably pretty high that you won't be the only person singing it.
6. Is this song in my range?
If the answer to this question was "No", I would avoid singing that song.
The most important thing is that you are comfortable and confident with your audition song! Whatever you choose, make sure you sing it well.