Capture a good angle
Instead of taking the picture head on, experiment with different angles to show off your features. If you turn your head a few degrees to the right or left, your features will appear less flat. Holding the camera slightly higher than your head so that its pointing down on you will make your eyes look bigger and help you avoid pig nose. Here are a few other ideas for finding a good angle
Know your good side and take the photo from that side of your face. Its the side of your face that looks the most balanced and symmetrical.
Angling the camera slightly above yourself and taking a snapshot of your face and chest will highlight your cleavage. Since this position is fairly unnatural, odds are, you already know what the camera will focus on when you take a selfie like this.