I think the problem with novice writers is WHEN to describe your characters, if at all. It sounds really stupid when a first-person narrator stops in the middle of a scene and says, "You know, I am blonde with blue eyes and a really good-looking guy with dimples to spare." Why? If it's important information then the readers should know as soon as possible, but when? In the middle of what scene? Why does the narrator stops and talks about his looks?
Even if the narrator only "refers" to his looks as in: "I washed my face and noticed a tuft of my thin, blond hair in the sink," we still wonder -- why is the narrator mentioning "thin, blond hair"? Why not just "tuft of my hair"? Most people wouldn't say things like: "I noticed my jet-black, wavy hair." It just sounds very stupid.
However, there are clever ways to describe your narrator without breaking POV or sounding stupid. For example: "She kept staring at me, and I suddenly became self-aware of the pimple on my face, and the thinning hair on my head." The point is, keep with the scene, action, and characterization.
Third-person narrative has similar issues. When does the narrator stop and describe the character's look? And why? And if it's close 3rd person, then the author has a POV issue:
Mary brushed her fingers through her dirty blond hair.
Bad writing notwithstanding, there's a subtle POV violation, even though Mary KNOWS she has dirty blond hair, because she isn't really looking at her hair and noticing: "Ah, my hair really is dirty blond." Descriptions like that work better if the narrator is clearly separate from the character. In a "close" 3rd person narrative, where the narrator's voice is the same as the character's, the author must take care not to violate the POV by observing characteristics that the character won't.
Avoid cliches such as looking at a reflection in the mirror or a pool of water. There's nothing inherently wrong with the mirror device, but cliches are always boring and, sometimes, laughable -- reminds us of the cheesy B-movie moment when Frankenstein's monster looks into the river and goes "Errrgghhhhh!"
Some other neat tricks to describe pertinent information about a character:
- Use action for characterization: I slid my palm over my face, grazing over the long scar on my left cheek. The scar my father branded there. The scar that seared not only my flesh, but also my soul.
- Use other characters or dialogue: She noticed the long scar on his face, and wondered what had happened to him. "You hair looks very nice today," he said nervously, "like ripples of sunshine."