It can be a bit of a conundrum ... but it doesn't need to be. If you have a uniform or a branded shirt for what you do then go ahead and wear it.
If you are in a formal setting then gent's wear a suit, ladies a smart dress or top.
But here is the trick, if you want to get good value from your head shot session then by all means bring a change or two.
Gent's a different tie or jacket. A shot with a tie one without.
Ladies ... change accessories, a jacket or top change is also an option.
Why ... well why leave a session with just one image you can use when you can have several. You can rotate them for different purposes, use them for different platforms but having more than one headshot will allow you flexibility and longevity.
There are 3 main options here .... Studio, in your office location, outdoors
The studio. If you want a clean crisp background, it doesn't have to be white or black, then the studio is a fabulous venue. It also has the added ingredient of studio lighting.
Your office environment, doing what you do in the location that you do it is a good place to start. It gives the viewer an insight into your work space and maybe others in it or how you work. With great lighting all locations can look fabulous.
Outdoors, if you want to portray a more relaxed less serious persona then outdoors in a local park in a number of different environments could be for you. This could also equally be you on location showing how you work.
Looking good and smiling on camera is not as easy as you may think. You also need to think a little about the purpose of the image and how welcoming or serious it may need to be.
The smile maybe dictated by the location and what you are wearing.
Overall when we take a picture together then we are going to be enjoying ourselves, chatting whilst shooting. As long as the image is natural a smile, or even a laugh might be suitable.
In some cases we'll have a make up artist to make sure you are looking your best. Having a make up artist makes the session easier for you and quite possibly a bit more fun.
Most important for women, or men with longer hair, is to keep it neat and under control. It can always look more messy in shot than in real life.
Women make sure your roots are done, eyebrows are shaped etc. Men have a shave or trim the beard.
Follow all of this and the pictures will be great.