Dress comfortable. When you and your family are in clothes they feel comfortable in, the photos turn out that much better. Try not to wear clothes that constrict too much or are made from itchy materials. Take it from someone who's been photographing people for years: the discomfort will show.
It is best to choose solid colors and avoid patterns as they can be distracting and actually take away from the image rather than enhance it. Dress everyone in a similar colors or complimentary colors. Choose one outfit, such as mom's and build the other outfits from there. Layers work well for Fall. A nice shirt layered with a cute vest, button down with a sweater, a dress with boots and a cute jacket, etc. Colors that work well for fall are
Have fun with accessories too! Boots, vests, hats, sweaters, a cute coat and scarves can be great for Fall portraits too!
Make sure your nail polish is nice & fresh
Make sure shoes coordinate with your outfit(s)
Check your pockets. Remove keys, wallets, cellphones, etc. before your session