I had fun with Project 333 this fall, in spite of the fact that I broke all the rules, kept finding new things all season at the thrift store (volunteering is bad for that), pulled out half my summer clothes again in a burst of KonMari, and figured I was finished with tiny wardrobes since I was obviously not very good at sticking to minimalism. The fall/winter page here was also a mess since I kept crossing things out and adding other things.
So I changed my mind and added a just-winter page. The clothes aren't new; but the list is cleaned up and easier to read. Strangely enough, I don't have many more items on it than most respectable "tiny wardrobes." It may not look like anybody else's version of what people "should" have, but it's working for me.
I don't know how most other people go about "thrifting" wardrobes, but here's what I do. Take what you can use.
I've seen articles online talking about which things you should snap up at thrift stores, and which ones are not usually worth your time. I'd say, make up your own list based on your experiences. I've read that it's impossible to buy used pants (because of fitting issues), and that t-shirts and sweaters are usually worn beyond bothering. But I take a pretty standard (although petite) pants size, and if I see the magic number on a label, it usually fits. As for tops and sweaters...well, you have to look past the poor ones to find something you do like. So figure out your own must-visit categories, those you'll look at only if you have time, and those that are a no-go for you. My quick-looks are the bargain rack, for obvious reasons; shoes, because although I rarely find any that fit, the odd exceptions make a regular look worthwhile; and dresses, because I like them and they're often more interesting than the tops and pants. If I have time, I look at the accessories. Anything else just depends on what I don't have much of; maybe I really need pants, a new purse, or whatever. Is there anything I wouldn't look for at all? I rarely look at athletic clothes, yoga pants and things like that. I also don't spend time looking at very fancy dresses. Jewelry was another section I used to skip, but this fall I've started looking at necklaces again.
The point is, your must-try list will be different from mine. It depends on what you love, what you do, and what you need. And one other "what": what you would like to try, or change. Maybe you have never ever worn jeans, or dresses, or high heels, or anything bright red, and it's just time. Maybe your secret wish is to be Rhoda Morgenstern. Thrift stores are the perfect place to try out new things.
Watch for colours you like but which may be hard to find in the average mall store. One idea I've seen online, but haven't tried, is to buy a too-big t-shirt in an interesting colour, cut the body away from the neck and sleeves, and wear the resulting big loop as an infinity scarf.
The last bit of advice is an obvious one, but...thrift stores vary. Donors, merchandise, and prices vary. I used to visit one store that always had great books, but horrible clothes, because the donors were mostly frugal older people who wore their clothes to shreds. Some stores sort things more carefully than others do. Some places smell better than others. If your only thrifting experiences have been bad ones, remember that better things may be hiding in another neighbourhood, or the next town.