Many times, when we think of getting pictures taken, we think of dressing up in uncomfortable clothes so you can spend an hour getting pictures taken for your annual Christmas card. My philosophy: Wear what makes you comfortable! There's nothing worse than being uncomfortable for an extended period of time. You want to feel confident and comfortable in what you're wearing. It'll reflect in your photos.
Be you
This may seem odd, but just be you. Wear clothes that match your family's personality. Many times people will go and buy new clothes, but if you don't plan to wear them for anything but these photos, is that really you? Wear what you'd typically wear. Your friends & family know you best that way.
Be mindful
So you're planning what to wear for your photos. You know where you want to take them, and being outdoors you know there will be a lot of earthy colors. Be mindful of this when planning outfits. Fall colors and an electric yellow shirt may not go well together. Neutrals, prints & patterns, solid colors, etc. are a safe bet. Accessories shouldn't detract from you either.