To obtain the best voice projection, you need to make the most of your breathing tools -- your lungs and diaphragm. To get the extra oxygen power necessary for powerful projection, you need to practice deep abdominal breathing. Practice by placing your hand flat over your belly button and breathe deeply so you feel your hand pushing outward. If your chest expands instead of your belly, your breaths are too shallow; you're not filling your lungs to their capacity. Inhale and exhale slowly several times to get used to the feeling of deep abdominal breathing. Then, place both hands on your belly, with your fingertips touching. Breathe in slowly and then exhale quickly, making a "huh" or "huff" sound. Practice making this sound and different sounds to learn how proper abdominal breathing affects how your voice projects.
Resonance is the quality of the voice to sound deep and full. When your voice resonates properly, you can feel the vibrations in your chest. Practice this by humming or saying "mmm-hmm" in different pitches, high and low, paying attention to the way the vibrations feel in your chest, head and mouth. Notice the differences in humming with your mouth open slightly and closed. When you can feel the vibrations strongly in your chest, your voice should resonate the most. Another way to consider resonance is to plug your nose and say a few sentences, using as much vocal force as you can. Then, unplug your nose and repeat the same sentences, noticing the difference in the way your voice resonates. Practice these techniques in order to get a feel for how to most effectively make your voice resonate.
While you don't want to be monotone, it can help to find your optimum pitch. You can do this while practicing your deep abdominal breathing. Place your hand on your chest and breathe deeply. Exhale, letting out an "ooooooooh" sound, starting at a high pitch, sliding down to the lowest pitch you can muster and then back up. Notice the point where you can feel the vibrations strongest in your chest; that's your optimum pitch. Of course, you'll need to vary your tone; you'll be able to project better if you practice delivering your most powerful vocals at or around this pitch.
Letting your personality and passion for acting shine will naturally allow your voice to project. Although you might be nervous for an audition, the more confidence you exude, the better your voice will project -- and the more you'll impress the director or casting agents. Practice your lines so when it comes time to audition, you'll be able to deliver them with authority and grace, allowing the unique quality of your voice to sparkle.