My kids won't sit still. Solid colored clothing with long sleeves generally serves to eliminate distractions in a portrait and place the emphasis on the subject's face. Wear classic styles of clothing.
Trendy fashions tend to date the portrait.
When two or more people are photographed together have all subjects wear similar styles in the same color family. This helps place equal emphasis on individuals and eliminates the possibility that one person will dominate the portrait. Portraits should make a statement about the subject. Having portraits done in an environment that reflects your lifestyle and interests is a great idea because it shows people what you're all about. When environmental portraiture is made outside the studio your photographer will schedule the session at the time of day when the light is most flattering to the subject.