1. You're using wire hangers when you should be using felt.
Wire hangers aren't supportive enough for heavier fabrics and will warp the shape of your clothes over time. Invest in felt hangers instead, which don't leave little points in the shoulders of your shirts like wire hangers do, have enough grip to hang thin, silky fabrics, and are thinner than plastic so you can fit more in your closet.
2. You're hanging your sweaters like you would a normal shirt.
Gravity will stretch out the neck and shoulders of heavier sweaters this way. If you don't have any drawer space, fold your sweaters and lay them on a hanger like above.
3. You're storing your clothes in the dry-clean plastic bags.
As soon as you get your dry-cleaning back, take your clothes out of the bags. Fabric needs to breathe, and the plastic can trap odors and humidity. If your clothes wrinkle inside your closet, it's because you have the hangers shoved too close together. To prevent crease and wrinkles, leave an inch in between each hanger.
4. You're dry-cleaning your clothes too often.
Overzealous dry-cleaning can weaken the fabrics (and make you broke). If the tag says "dry-clean only," take it to the cleaners. If it says "dry-clean" (like most polyester, cashmere, and nylon fabrics), it's usually safe to hand wash it at home and laying it flat to dry.
5. You carelessly stuff your bras in a drawer.
Bending the cups out of shape will ruin the bra form. Instead, stack them inside of each other like you see at a lingerie store.
6. You're wringing the water out of your swimsuits by twisting them.
This will ruin the elasticity in the suit and make it lose its shape. Instead, lay the suit flat between towels, and press and roll the water out. Then lay it flat to finish drying.
7. You machine-wash and dry your swimsuits.
This will cause the dye to fade and the fabric to pill. Unless you absolutely need to use a washer and dryer (in which case only do it once at the end of swimsuit season), hand-wash your suits in tap water or with a little detergent, and lay them flat to dry.
8. You throw your sweaty workout clothes straight into the laundry hamper.
This causes smelly mildew. Before tossing them in your dirty clothes pile, hang them until they're dry.
9. You forget to fasten buttons, hooks, and zippers, or to turn your jeans inside out before tossing your clothes in the laundry.
Loose hooks will snag other clothing in the wash and cause pieces to stretch out. Clothes that are heavily dyed, like jeans, should be turned inside out to prevent the dye from fading.