Your photography session is an event and should be dressed as such!
Here are my best tips:
Wear something you are comfortable in. Wear something that makes you feel like a million bucks. Do not wear something you are constantly going to be adjusting or feel the slightest bit self conscious in.
Think about how the outfit fits when sitting down as well as standing.
Pay attention to how your undergarments show under your clothing like noticeable bra straps.
Keep your outfit choices neutral or earthy toned. Think shades of cream, tans, or light browns.
When in studio, creams/whites/tans photograph best over black/other dark colors. The black can contrast with the all white studio.
Avoid bright and/or neon colors!
Avoid sequin, glitter, and frilly dresses for girls (unless it's a glitter session, of course).
Avoid logos.
Avoid sporty tennis shoes.
Avoid matchy-matchy! Gone are the days of matching white tees and jeans, thank gosh! Instead, wear outfits that compliment each other.
Avoid too short of dresses for women.
Men should wear slacks or jeans.
Avoid large hair pieces for little girls. Keep them small and neutral.
Boys should wear chino shorts/pants or jeans.
Don't be afraid to tastefully add in patterns and textures. General rule of thumb is 1 pattern for every 2 solids. For textures, think knit sweater.
Thin striped patterned clothing can show up wavy in images.
Some sessions, like an engagement or a maternity session may allow you the opportunity for an outfit change. In that case, I recommend wearing a dressy outfit and one more causal.
Layer! Layering brings texture and mixed patterns to a cohesive look and can keep everyone warm during those colder months! Layer a maxi dress with a slouchy cardigan or layer a long sleeve sweater and stockings with a cute toddler skirt.