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 it's 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:
Be sure to smile or do something cheerful. A sad face or a frown will not help.
Know which poses are considered passé. There are certain selfie poses that became wildly popular and are now well past their prime. You can still contribute your own examples to the mix, but do it with a little self-awareness so people know you're in on the joke. Notable choices include the infamous "duck face," muscle flexing, pretending to be asleep, or pretending to be caught off guard by someone else.
Use your phone's back camera. Many cell phones have two cameras: one in the back, and one in the front. Instead of using the camera in the front to take selfies, use the one in the back. The back camera takes higher-resolution pictures than the front camera, which will end up taking a blurrier selfie. You'll have to turn the phone around, and you won't be able to see your face as you take the picture, but it's worth the trouble to use the back camera.
Don't use a mirror unless there's no other way to get the shot you want. The picture will appear in reverse, your camera will be visible, and you're likely to end up with a strange glare. Plus, your selfie could appear distorted, since mirror glass doesn't always reflect back a perfectly accurate image. Stretch out your arm, use your wrist to point the camera toward your face, and snap away. It might take some practice to get it right, but eventually you'll know exactly where to position the camera to make sure it captures your whole face (and never cuts off the top of your head).