Verbs
I once heard verbs described as the engines that drive a story, and this is very true. You have to be choosy when using verbs to describe a person, however. A string of whimsical verbs used to describe a serious character will only make him seem whimsical himself. I like to call verbs in description active description. Rather than using verbs to convey motion and actions, I use verbs to describe things that are usually still. Use carefully chosen verbs to describe physical attributes of the character; rather than "her hair was short" use "her hair swung jauntily at her cheekbones".
Swirling tattoos cover his arms, climbing up from the collar of his shirt to twist around his throat, the ends hidden by his tousled dark hair. I try not to look at him. He could make me happy. ~Bethany Griffin, Masque of the Red Death
Selective Adjectives and Adverbs
There's so much controversy over adjectives. You'd think this would be a simple topic, but the fact is, deciding how much of something is beneficial is almost never simple. You have to factor in the pros, the cons, the way beginners do it, the way the experts do it, and on top of all this is a hearty dollop of conviction that may or may not be misplaced. My personal opinion is that adjectives and adverbs are there to be used. Use them well and sparingly, but use them. I probably use far too many, but I'm still not sure how much that weakens my writing. Adjectives in a character description are almost unavoidable. Still, select them in the same way you would select verbs - spend enough time finding the right ones so that the description is tailored to the character. Another something I've learned is that you should almost never go with the first word that comes to mind. Your brain is lazy, and I can almost guarantee you that with a good thesaurus and a little time, you can find something better.
Word Choice
Think carefully about the impression you want to convey about the character you're describing. This goes hand in hand with Point of View or POV. Figure out how the POV character talks and thinks and their attitude toward the person being described. For instance, if they're angry at the person or something they're associated with, their view is going to be skewed. Then choose your words accordingly. Also, think about the POV character's level of education and his social class. This should be important in any part of a novel, but it features prominently in the way your POV charrie describes others. My advice for this: get yourself in character (wear a skirt or clutch a dagger if you must) and read it out loud, in your character's voice. Alone, of course.
Movement
Think about the shyest, quietest, sweetest person you've ever met. Think about how they move, how they hold themselves. Now think about another shy, quiet person, but this time they're grumpy and a bit rude. I bet they're much different in the way they walk, their posture, their facial expressions. Even within stereotypes, individual quirks can affect movement, however slightly. Incorporating these subtleties can give insight early on into your character's mood and personality. In the below example, even though there's a lot of motion around Corlath, the motions he makes himself hint that he is a king and will be treated like one.
Continuity
I think it's happened to all of us - you're reading along, and the author is describing a character. You're nodding, getting it so far, and then the author sticks a pair of lush sideburns on the skinny glasses-wearing nerd. What? you're thinking. Wasn't expecting that. While I think we all know the importance of avoiding stereotypes and cliches, stepping out of them too far can get you a confused blink or two or even than infamous "O.o" look. This also tends to happen with settings in fantasy novels, especially. They just get too weird to be believable, and ever after my image of the place or person is ruined. So when you think you've come up with an interesting twist to add to your barrel-shaped tavern owner or your slip of a violet-eyed girl, run it past some fellow writers too, and see if you get "that look".
Surroundings
Draw attention to the surroundings to reveal character physically by asking "How does the character fit in? How do they stand out?" Using contrast and unity, you can draw very handy parallels between the character and the people around them, the character and the setting, or the character and the emotional atmosphere. For instance, if you set your scene in a room full of throbbing music, flailing bodies, sparkling clothes and spinning light and you throw in a man in a black cloak lurking in the corner, you have an instant image and instant intrigue concerning the character and what he's doing there. Now imagine a fisherman, worn and leathery, his hair bleached by the sun and his clothes full of holes, hauling in rope on a wind-weathered dock. This character fits where he is. He's been shaped by his environment, and that makes a statement. If we can imagine the environment the character fits in, we can see the character. Can you guess which the below example is - contrast or unity?
Comparisons
Metaphors and similes. We all know and love 'em. Similes are my favorites, but I'm going to lump them all together and call them "comparisons" for now. This can tie in to surroundings if you compare the character to items from the setting. For instance, if you have a pale-as-death sorceress in a meadow full of daisies, you could compare her face to the flowers. They're about as different as can be, but it's a strong image because of that. That said, please be careful with your metaphors. Create them with originality and sensibility in mind. Visit The Bookshelf Muse and browse through the colors, textures, and shapes thesauri for some metaphor examples. In the excerpt below, the book is largely about the sea, so the comparison of Kir's face to pearl and foam is yet another touch of that sea.
Gradual Reveal
This is a subtle and beautiful way to introduce a character without dumping a bucket of description on the reader. I find that a combination of this and other techniques often works best. Work in details gradually, through dialogue, actions, and word choice (especially word choice, since the fewer words you have, the more they have to count.) In the example below, no description is given of Grandpa Beebe, but I bet by the end of the excerpt you have a pretty clear picture of what he looks like.
But he made no move to go. Instead, he squatted down on the beach, muttering, "Don't see why she's got to parboil 'em today." Then he took off his boots and socks and dug his toes in the sand, like fiddler crabs scuttling for home.~Marguerite Henry, Misty of Chincoteague
So tell me. What techniques have you found and learned to introduce your characters?