First, realize that even though the audition accompanist should be a good sight reader, there is still the possibility for error, particularly if there are logistical hurdles making the job harder for the accompanist. Make sure that your sheet music is clearly marked with a beginning and ending point, and that any cuts are very clearly marked. Avoid excerpting the piece in such a way that the composer has to follow first and second endings or DS al Coda markings, which could be missed in a sight reading.
As for the physical score, it is best to prepare individual sheets so that the pianist doesn't have to worry about page turns. I have had a few auditions that could have gone better if the book didn't keep falling shut on me. It is best to make clear photocopies for the accompanist, making sure that none of the piano part is cut off. Even though you are looking at the vocal part, the pianist may have a difficult time guessing what to play if the piano part is cut off - this has happened to me very often. Finally, tape your photocopies with the appropriate clear markings on pieces of card stock so that they don't fall over while the accompanist is reading them. If possible, you could even print the music directly on the card stock.
Then, make sure that any information such as tempo, style, and dynamic markings, that may have been cut off from your excerpt are copied to the appropriate places in the music. For example, if your excerpt begins on page 2, make sure to write the tempo marking from page 1 at the beginning of your excerpt.
Make sure to practice your excerpt with a pianist so that you know what the musical introduction winds like and to make sure you can follow the accompaniment throughout. When at the audition, take a moment to explain anything to the accompanist that might be a cause for confusion, such as a change of tempo or meter.
Finally, make sure to thank the accompanist! It will be appreciated!
If you have any other questions about preparing a musical audition excerpt, leave a comment!