First off lets start right off with what not to wear. Please don't wear matching outfits! There, I said it. PLEASE don't wear the exact same shirts, or dresses or jumpers or parachute pants or tutu's.. Nothing, nada. NEVER match. Ok. Now that I've gotten that out of the way, here's what TO wear. Complimentary colors. There. It's that easy! Here is a nifty little website I found to search for complimentary colors! Just change the menu to COMPLIMENTARY, and pick your main color, and it'll show you a list of color's that coordinate!
Pretty neat huh? Try to keep patterns to a minimum. I usually say depending on the size of your family, for every 3 people, have 1 patterned and the rest solid. So, if you have a family of 5 maybe 2 of them with patterns. And make sure those patterns does clash, soft subtle patterns.
Don't forget you can always accessorize to create great layers and textures!! Bows, ties, headbands, scarves, tights, suspenders, hats.. to name a few! Obviously we don't want to take away from your family's beautiful faces, so keep them small and subtle. As a rule of thumb, steer clear of NEONS. no no no. They don't look good in pictures, and they reflect on faces.
I also suggest, parents coordinate with each other then kids coordinate with each other. It will all come together in the end. Typically I like to do pictures of mom and dad together, so if you're wearing the same color, it doesn't work. Break up the patterns between kids and adults. Don't put 2 kids in patterns, maybe do 1 kid and 1 parent.
Be comfortable! You don't want big baggy clothes on in your pictures. I know kids grow super fast, but please get clothes that FIT them for their portrait sessions. Same with you. Buy to fit, and flatter! Wear shoes you love, and are comfortable. OR... nix the shoes altogether, feet and toes always match!
As far as couples go, I suggest one patterned and one plain. It helps with visual appeal. Girls, make sure your dresses aren't showing any of your lady parts. Save those for later ;) Wear something comfortable, yet cute. Don't try to wear something that's not you. You want YOUR personality to shine though in your pictures, so pick something you love and would wear again and again because you look stunning in it! Also, I love outfit changes. I adore them. So if you can't decide between a few outfits? bring them!! It never hurts!