Here's a few suggestions, in ascending order of subjective difficulty to the average HN user:
Smashing Magazine (which doubles as good for web design).
Study the color wheel, read analyses of ways to use it.
Fashion websites.
Go to lots of museums.
Go to art galleries.
Flip through 2-3 fashion magazines per month at the bookstore.
Read book(s) on how to dress for different functions.
Go to high end stores and try things on.
Talk to artists, designers, architects, painters, etc. about why things look the way they look.
Take some art class, either crafting/doing-based, or art history.
Go with some sort of visual artist to a museum, and ask their thoughts about paintings.
Get a girlfriend/boyfriend/significant other who is an artist.
Any of those would help. The more you layer on, probably the faster you learn.
Also, study fashion/clothing advertisements. Scrutinize and analyze why they chose what they chose... many fashion ads will be regarded as high art in the distant future, because immense amounts of preparation, lighting, setting, and skill goes into making them. Almost anything that's in one is in it for a reason. Look at it and ask, "Why did they choose this? What are they trying to say/portray? Why this background/environment with that sort of clothing?" It's not instant gratification on the last one, but soon you start seeing the patterns. It's kind of beautifully awesome when you do.