What to wear
Take your time to carefully plan your clothing for your session. Avoid matching outfits but coordinate color and style (casual or formal) of clothing that compliment each other.
Keep it simple! Too many different colors and patterns can make for a distracting photograph. Around four works well.
I highly recommend avoiding clothing with logos. They are very distracting and they date a photograph rather quickly.
Also, think about what mood are you trying to create. Bright colors tend to be more lively. Neutral colors (beige, white, black, khaki, grey) are more classic. Solid-colored clothing works better than patterns when groups are being photographed together but definitely work in a pattern or two. It's good to match the colors in the pattern to the solids that the others wear.
Layers look fantastic. Think sweaters over shirts, jackets, scarves, knit leggings on girls with skirts, etc. Think of where you will be hanging these photos in your home and what colors you have on the wall and in the room when selecting your wardrobe.
SHOES - Dark-colored shoes or no shoes* are best. Kids shoes can be very distracting with logos etc. Also, match your shoes to your outfit, casual, worn out, every day shoes with well thought out clothing can really take away from the look you are going for.