T-shirts and sweatshirts don't look very good.
For headshots and fine-art portraits: I recommend simple dark clothing with long sleeves.
For couple/group sessions: simple, solid colors that complement each other.
Don’t match your partner’s look, complement it.
For groups/families: don't all wear the same thing (eg white t-shirt and jeans). You can pick one person (like one of the children) and have them wear a nice outfit. Everyone else will coordinate with the child but not each other. So if you have a little girl with a pretty dress with blue, yellow, and black tones, partner 1 could wear something with yellow. Partner 2 can wear something with blue.
Probably best to guide your children in what they'll wear.
People often choose a light top and darker pants. Some contrast is good, but extreme contrast is not.
Avoid very bright, vibrant colors and complex patterns.
You may wish to think about where you will display your portrait. What colors would fit that location?
Makeup and Hair
There may be some retouching involved, but there's only so much I technology can do. Retouching is like virtual makeup, but the results are not as natural as those achieved with the real thing.
In other words, everyone should pay extra attention to their appearance.
Even men can benefit from a little foundation!
As with clothing, avoid bright headbands and hair clips unless that is the intentional look.
A little hairspray can help control stray hairs.
Bring some combs, makeup, and a compact mirror in case you need a touch up during the session. Wet-naps to clean up kids.
Props are great! They will add a new dimension to your photo. I want to reveal something about your personality, so please bring them!
you can find prop suggestions at my Amazon store.
Huge Scrabble tiles like this one if Scrabble is your favorite board game.
Take a look at some posing ideas at my Pinterest Page.