Here are my suggestions for preparing for your portrait:

Clothing
During your portrait shoot, you may change clothes as often as you like.  However, the more you change clothes, the less time you will have for taking pictures.  Whether you choose formal wear or casual, you should look relaxed and comfortable.  Your clothing should reflect this by not being too tight or too loose.

Darker clothing will tend to slenderize.  In contrast, light colors can add weight to the body.  Darker clothing also helps to blend a body with the background, so that the face is the most important part of the photograph.  Bright colors attract attention away from the face.  Plain white shirts and blouses under a dark jacket work well.  A scarf loosely tied around the neck also helps to frame a face beautifully when a collarless blouse or sweater is worn.  Neckties should be toned-down in color and have a minimum of pattern.  Avoid prints and patterns, short sleeve clothing and short pants.  When arms and legs are exposed, there is more flesh in the image than on the face. This can be a major distraction.

Proper necklines for a portrait are important.  A wide-open neckline tends to thicken the neck.  A neckline that comes up to the base of the neck such as a turtleneck or a v-neck top is the most flattering.  It tends to slim down the person and frames the face beautifully.

Overall, try to select dark clothing that is below the waistline and lighter clothing for tops.  For example, dark jeans worn below a light colored sweater.

Hair Style and Makeup
Consider hair styles that are simple and off the face.  Hair that falls down onto the sides of the face (currently very much in style) creates distracting shadows.  Also, bangs that come down too low will keep light from getting into your eyes.  Eye makeup should be blended with no sharp lines between colors.  Avoid white above or below the eyes. This will not photograph well at all.  Too much color above and below the eyes will take attention away from your eyes, rather than attracting the viewer to your eyes.  Foundation makeup should be blended at the jaw line, so that there is no demarcation between the face and the neck.

Eye Glasses
Eye glasses may or may not be worn.  I will position you to minimize reflections, but many times it isn't always possible to avoid all reflections.  You may end up with some reflections from your glasses in your final prints.  Using non-reflective lenses in your glasses are a big help.  If you do not have those types of lenses, you may try to borrow a set of frames from your optometrist.  You may also have your optometrist remove your lenses for the photo session.  Many will do this at no charge, just ask them.

Jewelry
The fewer and smaller, the better.

Group Portraits
Clothing for group portraits must be carefully selected to blend the bodies together.  Decide whether the basic tone of the clothing will be warm-toned such as reds and browns or cool-toned like blues, grays, and blacks.  After you decide what you are all wearing, be sure to follow all of the above instructions.

Props
Using props can really add to your portrait, but they can just as easily take away more.  Again, keep in mind that you don't want the prop to be distracting or clash with what you are wearing.  If you must use a prop, make it simple and be a part of the portrait, not the main subject.

Last thoughts
The look and feel of your portrait should be kept in mind when selecting your clothing and hair style.  If anything is too noticeable, it could easily become a distraction.  Clothing, jewelry, and hair styles that do not attract attention to themselves are the ultimate goal.  It could be the difference between a very successful portrait and just another picture.  Finally, your portraits are meant to be enjoyed for many generations to come.  Fad-type clothing should be avoided, unless you are going for that trendy feel or you are planning to have new portraits made annually.  You will feel very rewarded when you view your photographs.  All of your efforts will be worthwhile.