Drink warm herbal tea with honey. The liquid will warm your vocal cords and the honey will help to provide lubrication. This reduces the amount of irritation that occurs when the vocal cords vibrate.
Buzz the first five notes of a major scale by putting your lips together and blowing through the center. Start on middle C for women and an octave lower for men. Middle C is the white key next to the set of two black keys closest to the center of the piano. Play the next five white keys and then descend while matching pitch while buzzing with the piano. For men, start on the white key an octave lower by moving along the piano to the left and finding the next set of double black keys. Play the white key to the left of the first black key.
Sing perfect fifths to gently stretch your vocal cords. Start again on middle C for women and an octave lower for men. Sing up to the note five white keys higher and then sing back to the original pitch. Hold each pitch for about two seconds. Continue singing the same interval moving up by half steps. Moving from one key to the next on the piano creates a half step. So, middle C up to the next black key will be one half step; the black key up to the next white key will be another. Sing as high as possible without straining your voice.
Start five half steps below the highest pitch you could sing comfortably from Step 3. Sing down five half steps on the sound "hmm." This will gently stretch your vocal cords, increasing your range over time. Move up one half step on the keyboard and sing this exercise again. Continue until you can no longer sing any higher without straining your voice.