Never try to smile in the photo unless you are really overwhelmed with happiness or at least in a good mood. It looks false. Such an “emotion” makes you a mannequin with an unnatural look. If you pose too long, your face becomes tense.
Can't make a good shot? Rest a bit, relax and try again. Be natural and share only a sincere smile, then the photo will be doomed to success.
Do not forget about the background. The background can be much more interesting then you, especially if the photo is taken in a crowd: someone can easily ruin your selfie. When taking pictures at home, control the order behind you - nobody is interested in your creative (or not so) mess. The most successful selfies are selfies against the backdrop of nature or just in the street.