What to wear
Solids over prints
In general it is better to to use solids over prints. Large prints tend to be distracting from the face. The focus of any potrait is the face or faces. My vote is always a solid over a pattern. Patterns tend to be trendy and date your picture after a short time period.
Color over Neutral colors
Color makes you pictures more vibrant. I don't mean neon colors. Just basics colors are good; reds, blues, greens, yellows, and pinks. It enriches your portrait and keeps you separted from your background. Dark colors tend to blend in the background. Whites, browns, and blacks or okay, but they can easily make a gorgeous portrait just "okay". Just keep in mind of the end result you want. I once picked "neutrals" for my family portraits and the studio choose a neutral background. They were the most boring, bland pictures we have ever taken. Color is your friend!
Length of Clothes
Short sleeve or long sleeve is the question? Short sleeves are not recommended for everyone. Look at the season you are taking the picture? What are you comfortable with? Look in the mirror with the outfit you choose from the front and side profile. Do you like it and are you comfortable looking at yourself in it?
Avoid trendy styles. You don't want to look back and say, "what was I thinking?"
Don't be too Matchy Match
The 90's are over. Dont' dress everyone in one single color. People do no need to wear all red in a Christmas picture. Choose a palatte of colors for the family. Find colors that complement each other, but don't put everyone in the same exact color. It tends to be overkill. Three colors is a good start. Have everyone wear one or two of those colors. You want to look like you belong in the same family, but you don't want to look like triplets either.
This is not the time to break in your new high heeled shoes that you just bought. You will be walking around and standing most of the time. You don't want your picture to have painful smile because your new shoes torturing you every step you take. Wear clothes that is comfortable to you and your family. If your little one hates his sweater because it is "itchy" that is not a battle you want to fight during a session. Don't add more stress to your day. Everyone needs to like their outfit and feel like they look good in their outfits. Happy people take better pictures.
Yes, shoes are important. They will show up in the picture at some point. Sneakers are not a good choice. I like shoes to be nice but also blend in. Good or bad shoes, you don't want them to draw away from the face.
Dress for the Season
You don't to wear a sweater in the summer and pass out from the heat and you don't to wear short sleeve, paper thin shirt on a cold November day and shiver through your whole session. Be comfortable.
Just a few tips to keep in mind to make your experience more enjoyable.