You should avoid wearing clothes made of 100% cotton because they don't wick away moisture and can lead to skin irritation or infections. Baggy clothes can also be hazardous, especially if caught in the gym equipment. Similarly, you should avoid wearing jewelry that can get caught in machines or loose clothing that can obstruct your vision during exercises. Finally, it's generally a good idea to wear clean and appropriate attire that won't cause offense or distract other gym-goers.