Back to Basics - Don't Be Shy!
You can have everything else working for you - posture, breath, jaw position - but you'll still be selling yourself short if you don't do one more extremely simple thing: sing out!
I see it happen all the time. Students will work so hard, be doing everything right, know all of their notes and phrasing, but when they stand up to sing they hold everything back and all that comes out is a little squeak or whisper. All the preparation in the world will not make up for a lack of volume. From a director/teacher's standpoint, singing out even imperfectly will highlight errors that we can work on later, but singing correctly yet quietly will never be appreciated.
From a performance perspective, a lack of volume is usually read by an audience to be a lack of confidence from the singer, either in themselves or in the music. A performing can do every note and articulation correct but if they held back the volume the audience will think something was wrong. A powerful, seemingly confident voice can mask errors. If you seem confident in what you're doing everyone will assume that those mistakes you made were intentional or just the way the song was written. Even if you are naturally a very shy person, don't let others know that! To be a singer is to be an actor in at least some small way so make it appear you're confident and the performance will be perceived better.
From a technical standpoint, what most people think is "singing piano" is not done correctly. Singing softly correctly takes more energy and effort than forte does. If you hold back the volume your voice will fail to hit the core, mature sound and will sound weak or frail because not enough air is moving past your vocal chords. Your body will also tend to tense up to compensate for the lack of airflow which is something you never want while singing. Your body naturally wants to sing with some force or volume so don't hold back just because you're a little insecure.
Just like with a haircut where it's better to cut too little than too much because you can always cut off more, always sing out more than hold back because we can always turn that forte into a piano but we can't create a forte from half a "squeak"!