I'm just back from P-Con, where the amazing Juliet E McKenna gave a masterclass in how to write character descriptions. I'm going to try to get the high points across here.
No Crime Call Descriptions.
Do not describe your character as if they were wanted by the police. "6 foot tall, with brown hair and blue eyes, wearing torn jeans and brown sneakers" is someone who is wanted for armed robbery, not your hero.
Think of what you do when you meet someone new. You tend to make a snap decision about them, based on one particular feature, and afterwards you think of them as "The tall boy with floppy hair" or "the man with the comb-over" or "the woman with with baby puke on her clothes". Chances are, if you someone has asked you to recall height, weight or eye colour after that first meeting, you wouldn't be able to. But you would have a sense of personality from things like the baby puke or the comb-over.
As a writer, you give your reader a quick one line description to hang a character on, and as you develop the character, and your reader gets to know him, you can add in more details. It's always fun to turn assumptions on their head. The woman with the baby-puke might not be a working mother, she could be a single doctor whose last patient got sick all over her.
Always describe your character through other character's eyes.
This means that if a guy is looking at a girl, he's going to see the way her hair swings round her shoulders, or how her boobs fill out her sweater. He's not going to be thinking about the brand of her clothes.
A girl looking at the same girl, however, is very likely to notice that the clothes all come from Walmart and her eye-liner is way over the top. And a mother looking at that girl will notice the dark circles under her eyes and the fact that she's wearing the same jeans as yesterday.
Mention how characters stand in relation to other characters. A direct stare is confrontational, and is more likely to come from a man. Woman traditionally tend to make less direct eye contact and to look up through their eye-lashes. How do your characters stand when you introduce them?
Remember that men make statements with facial hair. Throughout most of history, beards were the default for men, being clean-shaven was only possible for someone with money and leisure. The way a beard is styled speaks volumes. Neat or bushy? Trimmed or wild? Natural or carefully styled?
Women make similar statements with their hairstyles and eyebrows. Throughout history, a woman's clothes often told more about the men in their lives than about the women. A rich man had a wife who wore expensive clothes, even if they were not to her own taste. A poor man's daughter wore plain clothes.
An interesting factoid: the more inbred a community, the uglier the people tend to be. People who combine the genes of two very different groups are usually very attractive.
Mention smell. HG Wells was short, old and bald with a comb-over, but he got the girls - because he smelled like honey. What does your character smell like?
Hands often tell more than eyes. Are your character's hands still or active? Rough or smooth? Manicured or natural? Hands reveal economic status as well as personality.
