How do you pose for professional headshots?

You should research good photographers first, make sure you go to one with a lot of headshot experience, and whose work you can see online.

If he/she knows what they're doing, they'll be able to get you to pose in the right way. That's their job.

The main thing is to look like yourself - no silly pouts, no huge smiles, no fake 'sultry' looks, no strange angles but with really good lighting with a plain background.