It's picture day! Hopefully your portrait session is booked with a professional photographer who is great with people and knows how to elicit the best expressions from portrait subjects. That's his/her job, but here's what you can do to help ensure success before you get there:
If you are the one arranging the session, determine ahead of time and confirm with the photographer whether
an environmental portrait (ie. in an office or other appropriate
place), or plain (eg. white or gray), or other textured (eg. photographer's painted muslin) background will best serve your needs and plan accordingly. If it's an environmental portrait you're booking, it will generally work best if the location is attractive and has some space, depth and texture -- for example a large room with wood or coloured paneling, curtains and windows, as opposed to a small room, filled by a big round table and chairs, with four beige walls and a white board.
On the day, commit as much time as is required to ensure sure you have time to get to the session on time and to focus on the experience once you're there. You'll need time to switch gears and relax if you want a successful portrait.
Wear something that fits properly (women in particular: something with flattering lines and shape but not too tight or revealing), isn't dated or trendy and is appropriate to your position. This may mean a suit for you, or it may mean whatever you wear in your particular business environment. It's generally advisable to stick to predominantly solid, neutral colours, and mid-tones (ie. not white or black, although a white or contrasting shirt underneath a suit jacket is fine). If you are a guy, it's best to wear a plain (ie. solid, not striped or patterned) shirt because dress shirts very often wrinkle thanks to the tension created by the collar and the tie, and those wrinkles can only reasonably be retouched out on plain shirts.
If you are a woman, knits are fine but cardigans are generally not as they don't tend to hold their form well enough which makes them look sloppy. And keep in mind that the viewers of your portrait should be noticing your face, foremost, as opposed to your clothing or jewellery.
If you have the ability to bring optional wardrobe, like a different jacket (different colour and/or lighter or darker) do so, and ideally bring the options neatly pressed on hangers. You never know how you and your wardrobe will look against whatever the background is and it might be handy to have a alternative in case tones or colours clash or the contrast between you and the backdrop is not optimal.
Still on the topic of wardrobe, find out if you are doing a head and shoulders, or torso, or full length
shot ahead of time, so you can dress accordingly. If it's full length you
may want to bring clothing on a hanger so pants, for example, aren't
wrinkled from your wearing them in the car, at your desk, etc.
Find out if there will be a hair and make-up artist. If not, and if you are comfortable wearing make-up you may want to consider going to a make-up counter at a department store and/or hair salon before the shoot. We all have good and bad hair days, but it's important to like your hair (as much as that's possible) on picture day.
Ladies, bring at least powder and lipstick for touch-ups. We always bring our powder, but if you have one you like and wear anyway, it's worth bringing. And if you wear lipstick, make sure you've just applied it before walking in front of the camera.
If you have lunch right before your shoot brush your teeth before your session, or at least take a quick toothpick break.
And while you're at your session:
Listen to the photographer...don't overthink the process...just do what he or she tells you to do and submit and commit yourself to, and embrace, the process.
Smile, but don't keep smiling! Breathe! Let the photographer do her thing and smiles should happen. You want to look approachable, and studies have apparently shown that a smiling face is perceived as more trustworthy.
Don't assume there's retouching included and do request retouching if you have the chance. Hopefully at least a basic level of retouching has been negotiated and included with the session, but budgets and 'packages' vary. If it's not, you may want to take advantage of any opportunity to upgrade and get the touch-ups so you will not be limited to using your portrait really small (so viewers can't see any flaws).
And finally, if you'd like someone else's input, here's a link to an article I like that beaks down "The Perfect Profile Photo":
Hope that helps! Have a great portrait shoot!
P.S. For examples of portraits I've done for some of my LinkedIn connections please see my LinkedIn profile and look for my 'project' titled Profile Portraits.