Many portrait photographers, like myself, will send clients a style guide on what clothing choices work best for portraits. Clothing is another aspect that helps to make the shoot a success and it is as important as background, lighting and posing.
The goal is to make clients the main subjects and to flatter and make them look their best. Clothes should complement the photo, not detract from it.
Some things to consider: wear solid colored clothing so that the emphasis is on your faces; keep jewelry simple; if getting a haircut, it is best to make the appointment a couple of weeks prior to the shoot to avoid the just-cut-haircut look.
A basic guide will have the above considerations and more but depending on the kind of shoot will determine the specific style guide needs. Be sure to ask your photographer if you have any questions on what to wear for your portrait.