Dont wait until the last minute to plan your outfits. Have everything selected and ready to wear before the day arrives- give yourself time in case you need to shop for any items.
Coordinate, don't match
Choose outfits that complement one another, not match each other. Try to include 2-3 neutrals and 2-3 "fun" colors. Feel free to mix patterns and solids until you get a "look" that you like. Also, you may want to use layers and/or accessories to help build your color scheme and add texture and dimension.
Start with what you have
I usually have mom pick her outfit first. Find something you feel fabulous in! Its easy to pick a color scheme and find coordinating outfits for the other family members from there.
Go for "timeless"
You'll see this alot on photography blogs, and its for good reason. Avoid super trendy clothes and hair styles so that you wont be cringing when you view your portraits 20 years from now. Its also a good idea to stay away from logos/characters/other motifs.
Consider your home decor
You probably plan to hang these photos in your home. Do you want them to coordinate with your room decor? This isn't necessary, but if its important to you, you may want to take it into consideration when planning your outfits.
Here are few ideas to inspire you!
If you have any questions, please feel free to contact me!