When we look at a photograph, our eyes go to the lightest part.  When that photograph is a portrait, we want our eyes to go to the faces.  Therefore, with a few exceptions, it is generally best to wear simple clothes in medium to dark colors.

Casual clothing suits the outdoor portrait style and studio portraiture can be either formal or casual.

Your choice of color should be the best color for you, based on your eye, skin and hair coloring.  Also the location where the finished portrait will hang should be a consideration in selecting clothing colors.

If you are having a group portrait, you will want to coordinate everyone's clothing.  You may decide to wear matching outfits (e.g., navy turtlenecks with khaki pants) or coordinate clothes around a common color theme.
Solid colors in the same tones are best for any environmental portrait. These clothing choices will convey the "family" connection.

Make-up should be natural.
Hair should be styled as you see yourself -- your portrait sitting should not be the time to tryout a new hairdo.

Men who tend to have a 5 o'clock shadow may want to plan a quick shave before the portrait.

If you always wear eyeglasses and you have the type that darken in sunlight, bring another pair so your eyes are visible in your portrait.