Find a photographer you mesh well with. The photographer should make you comfortable and work hard to get you the kind of headshot you want.
Look at the photographer's previous work. See if the photos are good and if any of his clients found success in the model or acting world.
Find clothes you think you look good in. Not only will you look good, but you'll also have more confidence during your photos, which will lead to better headshots. Bring several different clothes to the photo shoot. Stick to simple colors, instead of busy patterns and designs. You don't want anything to distract from your face.
Take care of your skin, especially your face. After all, your face is the main focus of a headshot. Take care not to get sunburned or overly pluck your eyebrows.
Get in shape. Similar to taking care of your skin, take care of your body. You'll not only become healthier, but also be more appealing to people who might book you for modeling or acting work. You don't have to be super skinny. Rather, be comfortable with who you are.
Practice your expressions in the mirror. Try to work up a wide variety of expressions, such as business, confident, carefree, sexy and more. Consult professional headshots for ideas. If you want, have a friend take some photos of your expressions to give you more practice before your photo shoot with the photographer.
Come to the photo shoot well rested. You don't want bags under your eyes. Being tired can make you cranky, and you want to be as relaxed as possible.
Listen to the photographer during the photo shoot. Following his instructions will keep it an enjoyable time, rather than stressful. Ask questions and share concerns if necessary. Be relaxed and have fun.