Find songs that are tailor made for your voice. If you're a soprano, don't choose a song in a lower key just because it's popular. Browse song catalogs and listen to CDs til you find a song that's perfect for your vocal range and personality.
Ooze charisma. Unless you've entered a straitlaced operatic competition, popularity as a singer increases with a jovial personality, quirky hairstyle or sexy demeanor. If you have as much (or more) charisma as singing technique, you'll have a better chance to win over judges and audience members.
Hire a singing coach who specializes in competitions. With the TV success of "American Idol," there's a spike in the number of teachers handling competitive singing. An experienced coach will help you choose the right song and focus on impressing the judges.
Be original. Copying another singer's style won't work in a singing competition. You have to wow the judges with your own approach. Sing and move from the heart, and you'll have a better chance of winning.
Emote. No matter how good your singing voice may be, if you don't show much emotion, the judges won't be moved to vote for you. Express the message conveyed in the lyrics.