it is also a good idea to take a deep breath before singing , it sends oxygen to the brain and helps relaxation ,yawning is also good    When you yawn, the larynx (a.k.a. voice box) lowers. Lowering the larynx a bit creates a larger resonating space above it. (Lowering it too far doesn't help the sound, and actually can cause strain, so we often aim for "letting the larynx sit comfortably low"; its position at the beginning of a yawn is more appropriate than its position during a yawn.)
    When you yawn, your soft palate rises to close off the nasal passage. The nose, being full of hair and mucus, is a terrible resonator, and closing it off lets us make better use of the pharyngeal resonating space (i.e., the back of the mouth).
    When you yawn, the muscles at the sides of your mouth are relaxed. Avoid ice as it contricts the vocal chords