In brief, whatever makes you, your family, friends or significant others feel most comfortable and confident.
But I have a few subjective pointers which I would like to share with you.
1. Try to avoid matching outfits
Most families don't typically wear matching outfits in their everyday lives, so rather avoid this if possible.
We are all individuals with distinctive personalities and we should wear outfits which reflects this.
2. Add some colour
Don't be afraid to inject a dose of colour into your wardrobe. Muted neutral tones may blend you into the background depending on the location. As an example: Greens at a park and khaki for a seaside shoot may not be ideal. Also, try to wear colour combinations which compliment each other.
3. Patterns and logos
Try to avoid clothing which incorporates bold patterns as patterns have the tendency to lead to distortions in the image which were not originally there. Clothing with prominent logos may be distracting and should be avoided as well.
Less is more.
It is our job as photographers to bring out the best in you. So unless you are going for an extreme look, rather keep things simple and light.
Lastly, any photographer worth their salt will assist you with any questions you should have regarding what to wear on the day. Remember, the entire process should be fun and enjoyable from start to finish.