A good character description does not stop the story. Instead of a mirror scene, show your character's appearance through encounters with other characters, through what is out of the ordinary, and through their own thoughts, words, and actions.
In fiction, you want to draw your readers in and give them a vivid picture of the world and the situations you have made up. One of the hardest parts of creative writing is describing a character. Describing what the protagonist or another character looks like can quickly sound clumsy or overused. It is even harder when you write in the first person. These tips help you write a character description that lets your readers "see" your characters.
The mirror
What is the most obvious way to describe a character when you write in the first person? Let your protagonist look in a mirror. That is rather bad advice, though. Here is why.
If the mirror is the first idea that comes to mind, that alone suggests the device is overused. It is not subtle either. A mirror scene gives you a perfect chance to add a snapshot of your protagonist to the story, but it also interrupts the flow of your storytelling.
The mirror also reminds your readers that they are, figuratively, being shown something, and that this is exactly what you intended. That keeps them from forming their own view of the character, because they may feel pushed to see your protagonist in a certain light. To build a deeper connection between your readers and your main character (and the rest of your cast), describe the character's appearance, style, and other traits in a more subtle way.
Encounters
Another good way to show what your characters look like, without being too "in your face", is to put your character into an encounter. That way, you can contrast your character with another one, or point out features and traits that the two share.
This method is less forced than the mirror, but a description written this way can still feel forced. When you meet your rival or your crush, it is normal to reflect on yourself a little. How does my hair look? Is there a stain on my shirt? Am I overdressed?
Even so, too much of this thinking can interrupt the story. Your protagonist may worry that her hair is a mess, but she will hardly think about its color and length: those are far too familiar to her. In the same way, your protagonist may worry that he is not dressed up enough for a date, but would he really describe the color of his pants and the pattern of his tie? That is not very likely.
Unlike the mirror scene, an encounter that makes your character reflect a little can give the reader subtle information about your protagonist. Still, use this device with care.
Special vs. everyday
The best advice is to keep the description out of the way. A snapshot, whether a mirror scene or another detailed description of your protagonist, interrupts your readers. And is it really necessary to point out every little detail about your character?
It is human nature to notice what attracts or repels us when we look at someone else. But how often do we notice things about ourselves that do not stand out? Describing the everyday clothes or the usual appearance of a first-person protagonist makes little sense.
So it is your job as the author to decide when your protagonist is in a special situation that calls for a certain hairstyle or fancy clothes, or the opposite: maybe your character has to dress casually for once. In such a situation, the character may have thoughts about their appearance, or certain needs. When you point out these differences from the usual, you also give your readers a subtle, indirect impression of what your character normally looks like and wears.
Be your character
Whichever way you choose, keep one thing in mind: you as the author should not be visible to your readers. If you speak, think, or act, do so as your protagonist. Become invisible in the story and let the protagonist explain things. That way, your readers get more involved in the story, they understand the characters' motives and experiences, and the flow of your storytelling stays undisturbed. If you are unsure which perspective suits your story, read about first, second, and third person point of view.
To do this, do not rely on pure description. Dialogue can describe a character too. Instead of freezing the scene, describing it, and then carrying on with the action, keep the story moving: weave the descriptions into what is said, what is thought, and what happens. These tips for writing dialogue help with that part.
An example
To make this clearer, here are two ways to describe the same encounter. The first one:
I was nervous, a strand of pitch-black hair sticking to my forehead. Irritated, I tried to brush it away. Mika looked stunning in her light blue summer dress and delicate silver jewelry. I, on the other hand, wearing my dark suit and a tie in discreet purple, felt completely overdressed for the occasion. She didn't seem to notice, though, and gave me one of her stunning smiles that brought even more color to my cheeks.
This example is not bad, but it is not perfect either. The descriptions seem a bit forced and unnatural, especially from the point of view of the male narrator. Now consider this more subtle approach:
I was nervous, and it must have been obvious from the fine sweat on my forehead. Irritated, I tried to brush away one of the pitch-black strands of hair that stuck to it. Mika looked stunning, the light blue summer dress underlining her soft and cheerful nature just as much as the delicate silver jewelry. I felt completely overdressed. Maybe I shouldn't have chosen the dark suit with that tacky purple tie. But now it was too late for such thoughts.
"You look great," she said softly, giving me one of those stunning smiles that brought even more color to my burning cheeks.
In the second example, the descriptions of the characters' appearance are not crammed together as they are in the first. They come through the narrator's thoughts, and a line of dialogue loosens up a scene that was quite stiff before. For more ways to show what a character looks like, see the best ways to describe eye color in writing.