Keep it simple
"Less is more" isn't a timeless phrase for no reason. Less is definitely more! Keep your look minimal but elegant. Patterns are nice but keep them simple and consistent. Choose colors that compliment each other and even go along with the season your shoot is in. If in the budget, find clothes made with natural fibers to add to the luxury of the photos. Most importably, stick to what you know and stay comfortable!
Colors
If you are having a family photoshoot, coordinate with 3 colors. A lot of families choose chambray, cream, and navy blue during the summer months and then maroon, cream, and khaki during the fall months. You really just want to pick colors that most people already have in their closet so that the ones who are just going to throw something on will still match well!
If it is just you and a significant other, just pick colors that coordinate. You can wear a pattern and the other can have a solid color. What looks good with the season and time of day your shoot is at? Just consider the whole aesthetic you are going for and dive in!
Choose natural fabrics
I can not recommend this point enough. Synthetic fibers shoot differently in natural light then natural fibers. It really gives an expensive look to your outfit. If I was to splurge it would be on a beautiful linen dress and hat combo with gorgeous sandals to compliment.
Be comfortable
Do not just pick something out because you saw it on Pinterest and need that same photo! Don't get me wrong, Pinterest is perfect for finding Inso, but if you don't normally wear pant suits, don't start now! Keep it still about you and what you love to wear! It will feel the most authentic.