This is a great question and plays a big role in how your photos turn out. To make it easy and fun, here are my top tips:

Stick to similar tones without overly matching (avoid everyone in blue jeans and a white t-shirt). Neutral colors are always a good choice – use 3-4 neutral shades as your base, allowing everyone to coordinate their outfits (e.g., grey, tan, cream, blue, lighter brown, and white).
Primary colors or their variations work well (e.g., yellow/mustard, red/maroon, blue/navy/royal). When adding color, choose 2-3 main colors and one accent pop of color. Avoid fluorescent or extremely bright colors – strong colors can be great but not fluorescent. If you wear strong colors, make it a feature rather than the whole outfit (e.g., mustard jumper with denim jeans), and avoid having everyone in strong colors.
Try to minimize black – especially if one person wears a lot of black and another wears a lot of white.
For newborn shoots, stick to simple colors (grey, black, and white with pops of soft blues and pinks) and simple clothing.
Consider the location of your shoot – for a salt flats setting, wear “beachy” colors (e.g., white, cream, soft blues). For a shoot with green trees, consider colors that contrast with greenery, like warm shades of red, orange, or yellow, along with some neutrals.

Textures and layers look fantastic – consider layering a kimono or shawl over a tank top, using lacy or textured materials, wearing a textured beanie on a cold day, adding scarves and boots, or pairing t-shirts with jumpers, jackets, and vests.
Match the style of your clothes to the location (e.g., wearing heels and a fancy dress in the woods may look out of place). For lifestyle shoots, wear clothes you would normally wear at home without trying to match.
If you’re unsure where to start, pick one outfit you love and style the rest of the outfits around it. Ensure everyone feels comfortable in their clothes.
Show off your personality in your attire – don’t feel the need to dress plainly if your style is eclectic. You want the photos to represent the real you.
Think about your shoes – if you’re wearing a nice dress, pair it with wedges, boots, or heels instead of old flip-flops or sneakers.
The most important thing is to feel comfortable in your choices. There's no need to buy a new outfit, though it’s always a good excuse if you want one!