Gain sympathy. Every year, a sob story is told by a contestant about the personal struggles in his life. If there is anything in your life that will bring sadness to the judges, you will be known as the brave trooper who is reaching for your dreams against all odds.
Don't show up in a costume. One of the biggest mistakes people make is showing up in a silly costume and attempting to sing. Sure it might have worked in the first or second season, but the judges and viewers at home know people try to gain attention by doing such stupid things. It is a waste of your time and the judges' time, and that spot could be for someone who actually has real singing talent. Unless you want the judges to tear you apart, dress like a real artist or just stay home.
Serenade the judges. The ladies love a man who knows how to croon, so use it to your advantage. If you're a guy, sing a wonderful love song to Jennifer Lopez. Captivate her with your voice, and she will get all giddy because you sang to her and there is little doubt that you will make it through. Steven Tyler will also take kindly to a lovely lady serenading him. And if you have done extremely well, Randy Jackson and Jimmy Iovine might give you a yes.
Going as a group is a no-no. Chances are when entering as a group, one or two individuals might be picked -- a friend may have a mediocre voice and good looks, while you can sing at the absolute perfect pitch and not have the look. If one of you gets picked and the other doesn't, it could spell trouble. Remember, your friends are your competitors so don't take the risk of auditioning together as a group and risking your friendship.