When my SO and I first got together... I... there's no charitable adjective that comes to mind. He looked like one of those caricatures of geeky kids that get beat up a lot. Highwater khakis, short sleeved plaid shirts over faded tee shirts he'd owned since 9th grade, and oversized worn out sneakers of the sort usually worn in food service jobs. Does that sound just awful? If so, then I'm still not fully explaining the badness.
But I was not going to be one of those women that passive-agressively "loses" his favorite shirts, and I definitely didn't want to insult him or nag him either. So I waited until some of his old pants got a hole in them, and I took him to get a new pair. I said that instead of khakis, he might look at getting some jeans, and mentioned that I was sure jeans would look really great on him. He was ok with that, so we chose several to try on. I had him try on a few different styles, but nothing crazily outside his comfort zone. (no purple plaid skinnies here!) Some he hated, and some I hated. And then there were a few he didn't hate, and that I really liked on him. We bought those and I began to pile on the positive reinforcement.
Every time he wore them, I made sure to tell him how super handsome and sexy and amazing he looked. Really, I laid it on thick. And eventually he started wearing that pair of jeans several days a week. Pretty soon he suggested that maybe he really ought to buy a second pair of jeans, as some of his old pants (the ones he previously insisted were "perfectly good") were wearing out. Of course I felt like bursting into song, but instead I casually said, "Huh? Oh, sure dear, I guess we could get you some new pants." Now he wears his jeans all the time. Those awful old khakis still sit in the back of his closet, and I have never suggested tossing them out. He'll decide to do that himself some day.
I know some folks may feel like that's manipulative, but I don't view it that way at all. We're both happy now, and he truly does look gorgeous in those new pants.
TL:DR - positive reinforcement works wonders.