Hum quietly to yourself. You may pick a song you already know or make something up as you hum. Start with something in your lower range. A high-pitched piece can stress your vocal cords, which have only just begun to warm up after a night's rest.
Purse your lips as when you give a kiss and exhale in a long, firm breath. Allow your lips to flap as you exhale. These "lip trills" help warm up your lips, tongue and jaw.
Speak at a normal volume level. You may talk about any subject or use nonsense words. Speaking in your normal range can help you work up to more strenuous warm-ups.
Recite tongue-twisters at a normal volume level. These will help you coordinate your vocal cords with your lips, tongue and jaw. Start slowly and increase the speed of your recitation. Speak as slowly as you need to recite the tongue twister correctly.
Sing a song within the lower register of your vocal range. Sing gently and at a normal volume. This will warm up your voice without stressing your vocal cords with high-pitched notes.
Sing a scale run, up and down the scale. Begin in your lower register. Once you have sung the scale a few times, raise the scale a half step or two and sing the scale again a few times. Gradually raise the pitch until you reach the middle of your upper range. Decrease the scale a half step or two, sing the scale a few times, then lower the scale again. Continue this process of singing up and down until you feel warmed up. Do not attempt to reach the highest or lowest notes of your vocal range first thing in the morning. Your middle range should suffice to warm up your voice.