ALWAYS use a metronome when you practice!  If you do not have one, go to the links portion of the page and you can use an online version.  You will be graded in your audition on how well you keep time, and no matter how well you know the music or how well you can play it, timing issues can kill a line. You MUST be able to keep time, and the only way to make sure you are doing this is to use a metronome.  Good luck!