For those who haven't met you yet, your headshot is how they will visualize you.  You want to control that image by carefully choosing your wardrobe.   With a few exceptions, headshots should take the viewer directly to the face of the subject.  Your clothing and hair should provide a frame to your face.

Avoid patterns and logos.  They distract the eye.  But, don't be afraid of color.  It offers a bit of pop to your image.  Keep to classic, rich colors that don't distract.  

Accessories can help tell your viewer a bit about you.  Keep them appropriate for the image you want to convey.  A stylist might want to wear a more noticeable necklace.  An executive should keep it to something more sedate.  For men, the tie is often your statement piece.  

Avoid seasonal and trendy clothing.  To get the most out of your headshot, avoid dating it with clothing fads or limiting it to certain seasons.

Are you going for an executive look? A professorial look?  A stylist look ?  Dressing the part can promote your viewer's confidence that you are a professional in your field.

Avoid short sleeves, male or female.   

Avoid clothing that is too loose or too tight.  It's all the more obvious in a photograph.

And, avoid wrinkles.  Consider keeping your shirt/blouse and jacket hanging and change right before the session.  Unwrinkled clothing says a lot about your attention to detail in your business as well.

It is just human nature that you will be judged by your headshot :)