This is a great question. Seems simple enough but it's best to keep in mind what is appropriate for your industry or field of work. Most professionals should wear a suit and tie. While other industries might wear a polo shirt or a plain button up shirt.
When you hire Christopher Todd you get professional coaching as well as a pro photographer. This means you will be directed on how to pose from which way to tilt your head to whether you should cross your legs or arms.
Loud patterns are distracting and can date your photo. Skin tone colors should also be avoided. Beiges, peach, off white, shades of brown. You want colors that are contrast to your skin tone, but not too bright. Never wear wrinkled items. This will ruin your photo.
The short answer is yes. Everyone loves a friendly approachable face. However, there are variations of a smile that might work better for certain industries. For example, a doctor or attorney might want to smile but also look professional. A real-estate agent should be smiling and looking very friendly.
While certain items like a jacket or pants can be in black I would not recommend an all black outfit. Showing contrast in a photo helps create a a great image.