For large groups I always suggest to pick a color SCHEME and stick with it throughout. Warm tones or cool tones is often times just enough to get everyone 'matching' without squashing individualism.
In the above images you can see that the cool tones (blues, purples, black, white) were all maintained on everyone yet nobody matches exactly. In many cases people can wear whatever patterns they want as well. Stripes on one person doesn't exclude plaid on another. Bring a lot of solids in and then throw a pop of a pattern on the little ones! Or even a pop of a new color!
If you prefer a more uniform look for everyone, it can be done too! Pick a soft color though...all black or white will never photograph well, you'll end up being floating heads. Reds don't blend with other reds, and neither do oranges. Blues and purples and yellows and browns can be done hard or soft and still blend nicely.
For couples many of the same rules apply. For my own personal style of photography, I like to keep things light and simple and flowy. WOMEN...try a dress! I understand that we don't all sit around the house or go to work in dresses everyday, but it will do WONDERS for you in photographs and really express femininity. Getting dressed up always makes us feel pretty and put together so step outside your box a little and buy a dress that you feel beautiful in.
Keep in mind your surroundings. If we have decided on an urban session, get more funky and adventurous with color or patterns. If we are doing a sunset session in a field, stay light and soft. The weather may not 100% cooperate...so you may be a little cold or a little hot for a few minutes...but I promise you'll be happy you did it!
Newborns: These are some of the simplest sessions to advise. White and skin. Baby should wear a white onesie, (or pale blue/pink/yellow/gray) or else nothing much at all. Most of the session will be of baby in a diaper swaddled up in a blanket provided by me so don't stress over that part. Headbands are fine for girls...they help create something unique to your own session. Mom and dad WILL be involved in these photos as well though so be prepared. White, or black if you must, are the best options as any other color will reflect onto your baby and cast bad tones.
Individual/Kids: If your session is focused on yourself or your child...I encourage you most especially to create a styled shoot. Kids are excellent at playing it up for the camera and props really help to encourage that. Styling a session isn't difficult or time consuming and really pays off in the end. My favorite thing to do is create a story or theme for my clients and then run with it as much as I can. Styled sessions can be done with or for anyone!
Overall keep in mind your own photographers style and technique. As I mentioned earlier, I like to keep things light and bright. I use the sun in a unique way to capture it and play up the bright warmth that it provides. If I had to choose one word to describe my own tastes...it would be 'ethereal.' I love sunshine and dresses. I love to keep it real and natural. Next time you are wondering what you should wear for your own photo shoot, whether it be with me or anyone else, look through their site and see what other people wear. See what colors they utilize...do they pop bright colors or use muted tones? Photographers showcase their favorite images and styles so working to fit within the style of your chosen 'artist' will make your session go well, and make your photos truly a work of art for your walls!