I have 2 DDs, and *think* I have gotten the list to something workable. I actually make a list of basics and go from there, and copy the list of basics every year - everything else is gravy :) My kids are petite so lots of their clothes work for ~2 seasons, and I don't have to get new ones every year.
Basics are, for example for summer, a pair of brown khaki shorts/denim shorts, swimsuit, few basic coloured t-shirts, a few dresses, one matching tracksuit. Then I add as I see it (eg. tank tops, pretty t-shirts, hats, and so on). As long as they have the basics, we're good. Same for winter - basics are fleece-lined jeans, velour trackpants, long sleeved shirts, two really good hooded cardigans, then we go from there.
My thing is: always more tops than bottoms, and bottoms have to be basic and able to match a variety of tops. Some stores sell 'lines' of clothes where everything matches - sometimes I do that.
Also, for girls: accessories mean a lot, just like for grownups. Hats, scarves, necklaces, gloves, vests, boots...
ETA: I also do 2 BIG shops a year at the outlets and spend hundreds. That makes it easier for me to know what I have, and I can coordinate a bunch of stuff as well. I do a spring (April) shop and an August shop.