Shop local - and alone
Personal shopping is of course more sustainable than online shopping - especially if you are on foot and support local businesses. Clothing that is shipped around the world has a significant carbon footprint and is often packaged in plastic.
And it's also less likely that you will return clothes that you tried on locally.
Did you know that we buy on average 5 pieces of clothing a month (excluding underwear and socks)? That makes 60 new clothes in your wardrobe a year. A lot! You can further reduce your shopping frequency by turning down your friends from time to time when they ask you to go shopping together. Be strong, because these shopping actions often tempt you to buy things that are not really necessary. Similar to when you go to the supermarket hungry.