Don't.....
Slouch or hunch your shoulders
Move stiffly or jerkily
Drop or tuck in your chin when trying to sing low notes
Stretch your head upward when trying to sing high notes
Strain or push your abdominal muscles
Do...
Be relaxed
Keep your movements fluid
Keep your chin level
Keep your head up
Keep your shoulders relaxed
Keep your toes pointed forward with your weight on heels and soles
Keep abdominal muscles relaxed
Keep your back muscles relaxed
And Smile!