My advice would be to work relaxing your upper body and supporting from your diaphragm. My apologies if you already know this stuff!:
1. try holding a piece of paper against the wall, then letting go and keeping it there with your breath. If you're doing it right, you'll feel the effort in your upper abs, just below your ribcage.
2. imagine you're holding a heavy shopping bag in each hand - your shoulders will then fall back naturally and improve your posture
3. think of singing through a whole musical phrase or sentence, rather than taking it note by note
Hope that helps! I'm not a singer but I'm a woodwind player so I have similar troubles with projecting my sound too