I don't mind admitting that ever since she was an infant my girl has worn mostly secondhand clothes. She gets hand-me-downs from her cousin, who is a year and a half older, and I buy from thrift stores, children's consignment stores, and eBay. Generally I shop about one size ahead and toss items as I buy them here and there into a bin in the attic.
The problem with this strategy is that it is all too easy to wind up with too many clothes, duplicates (two plain black long-sleeve tees for a kid who rarely wears plain tees), and clothes that don't mix and match (adorable pair of printed leggings that go with zero dresses in the closet).
For a while I was keeping a tally of pants, shirts, etc. in each size on an index card that I slipped into my wallet. This helped a bit to keep from overbuying in any given category, but because I was just keeping track of numbers I still wound up with too many similar clothes, "missing" basics, and items that didn't go with anything else.
I have a new approach I have been trying out that seems very promising so I thought I would share.
The first step was to decide how big I wanted my girl's wardrobe to be overall. I decided on two weeks' worth of clothing -- about 14 bottoms and 14 tops for each season (cold weather and warm weather). This is substantially less minimal than the suggestions I found when I googled "how much clothing does a child need?" Most other sources recommend around one week's worth of clothing. I decided two weeks' worth is better for us because I do laundry once a week and especially in mud season (which around here lasts roughly forever) it's not unusual for her to go through two or even three outfits a day.
Next I tried to be honest about what kinds of clothing my girl actually likes to wear. For example she only likes "soft pants" so that brand-new, tags still attached pair of corduroy pants at the Goodwill is frankly a $4.99 waste. She doesn't like tights so there is no point in accumulating a drawerful of skirts to wear with them, no matter how adorable and inexpensive. Mostly she wears knit dress + leggings or knit pants + long-sleeve tee, so I planned around those outfit formulas (for cold-weather clothing, that is).
I realize, of course, that her preferences could change on a dime and a lot of this effort will have been wasted. But, my girl doesn't happen to care much about clothes in a fashiony sense, her main concern is comfort, and her preferences have been pretty stable over time, so I think this kind of planning is worth the risk in her case. And I think some of the ideas I'm sharing here could still be adapted for kids who have more definite and specific opinions about what they wear.
Step three was to come up with a rough strategy about colors etc. to buy in each category. Basically, for leggings and pants I'm looking for darker colors (practical, don't you think?) and solids. Dresses and tops get brighter colors and prints, patterns, or some kind of appliqué or embellishment. She's down with flowers, stripes, polka dots, and animals but (tragically) hates plaid. Purple, blues, and pinks are the most loved colors.
Then, I made a little chart with a square for each piece of clothing needed. When I buy something I fill in a square with the color(s) of the item. This way, I can purchase things gradually as I find them, but make sure my girl ends up with a mix of clothes that works together well.