This is the advice I give to my families as they prepare for their family session.
Clothing: I love neutrals or subdued colours. I strongly advice against loud graphics, strong colours or bold patterns on clothing. These things pull the eye away from the expressions or interactions. Jeans are considered a neutral. You can mix and match corresponding colors. If you'd like my advice, please e-mail me. If you need a little colour theory refresher, here is a simple explanation (scroll down to Color Harmonies for advice on mixing and matching). I love subdued colors so finding colours with tints and tones in them are ideal. As far as style of clothing, wear what you feel you look good in. If you want to minimize weight, dark colours are good for that. For women, here are some additional tips to help you feel more confident in your dress (although - I wouldn't call them "flaws").
For men, dress in a way that makes you feel comfortable and confident as well. Dark colors and blazers can minimize any weight concerns on men as well. Chances are, your significant other has some big plans for you in this department to complement the look of the rest of the family.
The advice that I lean on the most is to be happy. Getting dressed up for a family portrait can be a stressful event for families with littles or families with teenagers that may have their own ideas on what they want to wear. Keeping the preparation time light and upbeat is important. I cannot count how many families I have photographed that have rushed in looking pristine, but their kids are growly and mom and dad are stressed! I usually have to take some time to help the family to unwind so we can get natural smiles and interactions in our session. For me, having grown kids and knowing how I feel about our past family images, this family session is going to be part of your legacy. You will be looking at this family image and memory for the rest of your life with nostalgia of when the "kids were that age". It is more important to me to have their natural personalities shining through over what they are wearing. If they battle you hard on what they want to wear. Have them come to the session in their choice of clothing with a back-up set of clothing (your choice) in the trunk. I may or may not be able to help. We could take some in their choice of clothing and some in mom's/dad's choice of clothing. For me - happy family members is the #1 priority as we start our session.
During our session, we will have posed images as well as interactive images. My goal is to make it fun because I want the memory of this photo session to be part of the nostalgia you will remember it being years into the future when you see your family images.