Shhh. This one's top secret. Much as they might make a show of kicking up a fuss, men actually like it when you give them fashion advice. In fact, some of them come to depend on it. Women tend to have way more experience of clothes shopping and fashion magazines than men do, and they know how to use it. If it was left to men, most of them would spend their entire lives in jeans and t-shirts or suits.
If you can make them look cooler, smarter or sexier, or just generally help them to dress in a style that suits them, they'll really appreciate it. On top of giving them a fresh wardrobe, it also tells them that you have an interest in making them look good, and that you want them to look their best.