Take a minute to relax:
go through every muscle in your face, relax your impression, shoulder etc.
walk and stand tall
Make sure the light is right:
your face is evenly lit
the light doesn't throw any shadows under your eyes
Practice your impression infront of a mirror.
Contour and highlight with your makeup.
Practice a nice soft smile.
Put colour on your lips so they don't blend in with your face.
And remember: The camera doesn't add ten pounds, bad posing does!