Are there positions in which you should stand?
Flattering stances and poses can depend on the camera angle.
You can choose to angle part of your body to minimize curves if you wish as it does work. You can also pull your arms slightly away from your body and/or put one arm in a non-resting position in order to create a learner upper arm look. Season celebs on the red carpet tend to get quite good at this.
The camera can add weight. If you wear styles and colours that support and flatter your complexion and shape this is much less likely to happen. At very least it can minimize the effect significantly.
People often tilt their heads in photos (sometimes they are directed to do so). In a professional context. In my opinion, it is ideal to have the head is level with the camera (unless you’re going for a playful or artistic shot of some kind). A tilted head position can read as wavering and/or unnatural. Make sure your posture is confident yet relaxed.
Should you smile as widely as possible or is a softer smile preferable?
Practice in the mirror and take photos of yourself to find you best photo smile.
A genuine smile is important. For some it will be a softer smile and for other it may be a wider grin. Find your widest smile and then bring it in a notch to start and then compare smiles. Your choice of smile has to do with the structure of lips, teeth and gums as well as what feels genuine and comfortable. Often the most flattering photo doesn’t show your gums too much.
This (right) is the widest version of my smile that I find flattering in a photo. When I’m not being photographed, I don’t monitor or restrict my smile of course 🙂
What kind of light should you seek?
Natural light or simulated natural light is often the most flattering choice. Make sure to be well lit. Generally, shadows are not flattering (and not appropriate for business headshots) but they are sometimes it’s used strategically for specific type of creative headshot.
You can shoot outside, in a room with streaming natural light, or using softboxes/umbrella continuous lighting.