They want to see you! - Headshots vary from other portraits in that a headshot shouldn't have a lot of dramatic lighting and shadow. Casting people want to see you. That's the point. So acting headshots are almost always well lit.
Your headshots should look like you! - Agencies and Casting directors expect you to look just like your headshot and don't want to be surprised when they see you in person. So your headshot should have little or no editing done to it. Of course, we can remove temporary blemishes or that stray hair, but they want to see the person in the photo walk through the door.
Clothing - Keep it simple. Avoid logos, busy patterns, and graphics. Simple colors that work with your eye color and or complexion tend to work well. Consider your character type when picking your clothing. Don’t wear a slinky dress if you want to play the soccer mom. Bring multiple options to your shoot so we can pick the clothes that make you look right for the part.
Jewelry - Don't. You don't want anything that's going to distract a casting person from seeing YOU! They may only take a second to look at your photo. Make sure their attention isn't drawn by some item of jewelry. (The same goes for having your hands in the headshot.)
Eyes - They say the eyes are the window to the soul. You don't want your soul to be bloodshot and glazed over. Before your shoot, get a good night's rest. That'll give us a great canvas to work with during your shoot so you can smile with them, show depth through them or tell any other story you'd like. They are very much an actors toolkit. Let's show them off in your headshots!
A Little Research - Various talent agencies want different things in their headshots. Some are find with the top of your head being out of the picture, some are not. Some want grey backgrounds, some do not. If you're getting headshots for a particular agency, it's a great idea to know what they're looking for. If you don't know, I'd be more than happy to help you find out.
Makeup - Refer back to how casting people want to see the same person in the photo walk through the door. If/when you do your makeup, I generally tell people to go easy on it. The heavier the makeup, the more likely it is to cake and crack. Also, if you really want a makeup artist to work on you, I know a number of good ones I can bring in. I will only charge you their fee without adding anything on. But for acting headshots, you'll want to look like that when you go in for auditions. It's generally better to do light makeup yourself that you can easily recreate. Or.. not do any makeup at all.