Practice behaviors that lead to good vocal health. These behaviors include drinking at least eight eight-ounce glasses of water each day, not smoking and not over-exerting your voice.
Sing every day, but always warm up before you sing. Warm-ups should consist of five to 10 minutes of soft, gentle singing and vocal exercises. A good vocal warm-up exercise is to sing a syllable such as "la" on the first five notes of a scale, then back down. Start low, and go up one half-step after you complete each scale.
Drop your jaw and relax the muscles in your jaw, face and neck when you sing. This increases the area in which the sound can resonate and allows you to hit a wider range of pitches. Muscle tension can cause your higher pitches to sound flat and thin.
Raise your soft palate as you sing. This is the soft area in the roof of your mouth near your uvula. If you try to yawn, you should feel this area rise. Raising the soft palate also helps with range and pitch accuracy.
Visualize coming in from over the top of a note when you sing high notes, rather than "reaching up" for them. This helps you not to fall flat in your pitch.
Visualize sending your voice toward the back of the room in which you are singing. This exercise helps you to project your voice powerfully, which is helpful for auditions in which you do not have a microphone. Only do this exercise when your voice is properly warmed up.
Practice a wide variety of music. Although you may have a song picked out for your audition, your judges may ask you to do another song, or sing a piece in a different genre of music. Having a wide repertoire of pieces makes you a more skillful, prepared singer.