Coordinate but don't match.
What we mean by this is, for example, when using the color red, have one person in red pants, one in a red shirt and one a red tie. It is best to put one person in a pattern or print & pull everyone else's outfits/colors from that pattern.
Scarves, hats, vests, jewelry, sweaters, hair bows, head bands all add depth and interest to the photos.
Shoes are important!
Sneakers aren't the best choice when looking for photo ready footwear. Unless we are talking about a precious pair of converse that fit the mood of the session.
Seasonal dress matters!
Everyone needs to be dressed in the same season. If one person has on a sweater then everyone else needs to be dressed in winter attire, such as boots instead of sandals.