Take a trusted loved one with an iPhone to the park

If you're in Chicago, Millennium Park or Grant Park could do the trick, among others. My now-husband has taken my last two or three headshots on his iPhone. We find a good park after I've gone to a salon to get my hair done. I wear some type of button down, or nice dress. Makeup is basic, but nice. I like having some degree of foliage around, but that's just me. The last time my husband did this he took literally 200-300 photos in the park of me on a nice sunny day - it was a lot, but we parsed through them, and I found my top contenders. I also brought books to pose with.

Pro tip: Go look at the headshots of colleagues or people you respect who do work similar to yours, and determine what you like and don't like about how they present themselves to the world in their headshot. This helped me figure out my personal formula.