Stick to the industry-standard format - 8"x10" (20 cm x 25 cm) portrait.
Use a professional headshot photographer. Shop around and choose a photographer that you feel comfortable with.
Plain black or white clothes are best for black and white photography. No busy patterns. Bring a few changes of tops with you to the shoot.
You may wish to use a professional hair and makeup person if you are not confident in doing your own. Otherwise, keep your hair and makeup natural looking, as you would wear in the daytime/to interviews. Bring brushes, hairspray, makeup etc. with you for touch-ups.
Have one versatile shot for your general self-promotion. To submit for specific auditions, accentuate your type(s) in your appearance(s) for the headshot(s), but don't go over the top. As a lot of the differences should be in things like the way you are lit, discuss the looks you're after with the photographer.
Take direction from the photographer and keep your expressions honest.
Get opinions from industry professionals when choosing a headshot. Make sure that the headshot you choose looks like you.
Have your name and phone number (or agent's logo) printed on the front of the headshots or write them on the back. Name digital copies your first and surname (eg. "mary_ling.pdf" – not "headshot.pdf").
Duplicate your headshot on photographic (not photocopy or plain) paper.
Update your headshot regularly - around once a year and each time you get a new look (eg. a new hairstyle or colour).