Buying simple pieces that easily go together, and can be dressed up or down. Mid level of refinement as opposed to very casual or very dressy. Don't buy things that require you to buy more to make them work.
Inventory what you have, try things on to assess fit, try to come up with new combos. If you find wardrobe holes, shop with a plan.
Buying things that are not as size dependent, if your body fluctuates. Stretch is your friend!
Really looking at the quality of the clothes, the seaming and details, and checking for holes/pulled seams/stains if thrift/consignment.
Thrift/consignment is Much better in wealthier areas, but pieces will cost more. Accessories like belts and scarves are good choices. Buy Nothing without trying it on first- no matter what the cost.
Buy for the life you have and think of PUPY (potential uses per year). If you wear a uniform, or work full time, you need very few off duty clothes. If you have extreme weather, invest in the pieces you need. If you walk a lot, prioritize good shoes.
Take care of your clothes. Take shoes to the cobbler once a year, don't put delicates in the dryer, use Dryel sheets for dry clean things, wear easily washable things when eating messier foods, learn to do simple repairs/clothes dyeing.
Last one- Never Under Any Circumstances put clothes (or other nonessentials) on credit! A lot of stores are more interested in signing you up for a card than selling good products. That "10% off" comes with 29% + interest.