One of the biggest mistakes people make when asking for a raise is bragging about nothing.
Have you done your job and been an average employee? If so, that's not exactly a selling point. That's what you were hired to do and it's what's expected of you. A raise simply for this reason is basically you saying "Well, I'm not terrible and I'm doing the minimum so please give me more money." A raise means you're doing more, so show that you've gone above and beyond your job responsibilities. Showing initiative and putting in extra effort are the things your bosses want to see.