1. take deep breaths,from the diaphragm,this releases more oxygen to the brain ,and helps to calm if the performer is at all nervous.
2.I practice arpeggios.[singing broken chords] I also sing scales
but I do this regularly not just before gigs.I treat my voice like an instrument,I practice for hours on my instruments,and to a lesser extent I practice my voice.
there is another breathing exercise ,That I have been doing for years,I take a deep breath from my diaphragm,and then release slowly. Then I take another deep breath and release quickly[cleansing breath].
have a listen to my voice.http://www.dickmiles.com