I sold a story to the up-and-coming publisher Damnation Books where I was fortunate enough to have some input on the cover art.  My suggestion was a scene from the story where the three main characters—all teenaged boys—are looking down at a pistol on the ground.  To add impact, I suggested the image be from the knees down, making the gun the center of the image.  I began to describe what each of the boys would be wearing… and paused.  What would they be wearing?  And what would their clothing say about them as characters?
One of them, Jack, was easy since I’d already described him in the narrative:

If my brother was cool, Jack was ice cold.  He always wore jeans with holes in the knees and rock band t-shirts with half-naked women or profanity emblazoned on them.  Today’s shirt had a demon throwing a priest into the ocean.  His blond hair reached his shoulders in that heavy metal singer kind of way.

With that description, you have a good idea of who Jack is.
But what of the other two?
I’ve never been a fan of “character sheets” where you fill in excruciating detail about a character: favorite color, describe their first kiss, what’s their dream vacation, what do they wear?  It’s one thing to know your character well; it’s another to spend so much time on information you’ll never use, especially in a short story.  But here I was, thinking about their clothing.
It’s especially important since they’re teenagers—an age where we try to create some kind of image or persona, and clothing plays a large part of that: goth, preppy, emo, jock.  Would they wear sneakers or boots?  If sneakers, new athletic high-tops or an old, rundown pair?  If boots, tan work boots or black army?  As I decided what each would be wearing, the more I learned about them.  And, I came to realize, I’d missed opportunities to give them added dimensions with only a few extra words.
Now, I am not a convert to the character sheets, but the lesson for me is to consider what some of the questions are and though I don’t need to come up with an answer to every question, I should at least consider what the answer might reveal.