The basic rule of thumb for headshot clothing is keep is simple.  You don't want anything distracting from you. 

Patterns are really hard on a  lot of images.  It’s better to stay away from bold bright patterns in most cases.  Plus, the objective is to have the shots focus on you, your eyes, your communication to the camera as you shoot.  Patterns can often distract from that.

Iron your clothes and have them ready.  You do not have a lot of time between looks. You and your photographer are on a mission to get as many looks and great images.  If you are stopping the flow of the shoot to iron clothing it not only wastes the photographers time and annoys them, it also drains your energy.  So have your clothes ironed and ready to go.

Jackets are wonderful because they change looks rapidly.  You can go from business to edgy just by changing a jacket sometimes.

Baggy clothes add weight to your body.   Form fitting clothing always photograph better, even for individuals that are a little heavier than average.   

Unless you are stick thin spaghetti straps or strapless can often photograph making you look heavier than you are, even if you are a size zero.  I am not saying not to bring them or wear them. They are fabulous for layering, but know your body well enough so you are wearing them correctly.

Stay away from sports logos.  Sports fans are passionate people.  If you are wearing the wrong team you would be surprised how it would sway someones choices.  

Especially with kids stay away from trademarked images.  Don’t have micky mouse on a shirt or any other comic, cartoon or trade marked images like that.  All kids should wear NORMAL clothing.  No logos, no frills.  Mothers will little girls often dress the daughters up like dolls, but in this business that is absolutely not wanted.  This is not pageant wear.  Children should not be in their Sunday best. 

Glasses are a major challenge for photographers.  We need to have your eyes communicate to the camera.  many actors bring glasses for their shoots and some actors only wear glasses and hate contacts.  I recommend people with glasses pop out the lens for the shoot so the photographer does not have to contend with glare.  There are glasses that have anti-glare.  But I have learned there are different qualities of anti-glare.  Very few are legitimately so.  If you are a person that normally wears glasses and has anti-glare bring an older set of glasses with you and pop the lens out if you need to.  Obviously if you have a really fabulous pair of glasses you do not want to destroy them.  Maybe go to a cheap sunglasses stand and see if they have a cheap pair that match what you wear and pop the lens out of those.  No need to reunion something you need that is expensive to replace.