Coordinating outfits, not matching are ideal! For family shoots, I usually suggest that the mama picks her outfit first and then coordinates the others around hers. A mix of solids and prints look great as long as the prints aren't too crazy. Pull solid colors from the prints and try to avoid shirts with wording on them.
DON'T FORGET SHOES! Often times, people don't pay attention to the shoes they put on thinking that they won't be in the photographs. But I'm here to tell ya...they certainly will! And guys, cute socks do wonders!!
If you're considering getting your hair cut or colored before the shoot, make sure it's a few days before. That way it can grow out just a touch and you have plenty of time to get it fixed if something goes wrong.
If you wear make-up, apply it just a bit heavier than you would normally wear it.
Make sure your fingernails and toenails have either no paint or fresh paint.
Accessories are like magic! They can add little details to your outfits that make a big difference.
Plan for 2-3 outfits, so you have a little variety!