Danielle Ackley McPhail is the author of Yesterday’s Dreams and Tomorrow’s Memories, as well as the head editor on the Bad Ass Faeries series, of which I am an assistant editor. Also, she has a Live Journal at Sidhe na Daire. Here are some words of wisdom from her on the subject of writing description.
Description in fiction can be tricky. There are several different ways to approach it, depending on the situation.
For the most part, I prefer to weave my descriptive in among the action:
The room was dark as Kristy entered. She moved cautiously, running her left hand against the rough, papered wall, shuffled her bare feet carefully across the wood of the floor. The dust was gritty and harsh on her soles.
A few feet into the room she encountered a pole lamp and fumbled for the switch, not quite correctly anticipating where it would be, but finally finding a dangle cord descending from the domed shade. A sharp tug sent weak light tentatively into the dark, not quite pushing it back. She could make out the curve of an old-fashioned chest nearby, to her right, and on her left, a straight, spindle-backed chair, everything else was no more than vague contours.
This makes it immediate and from the character's perspective, rather than the author's and reveals things in pace, but it is for situations where the character takes precedence over location. Another thing it does is engage the senses in a way the reader can relate to, in effect, walking them bodily through the scene. Engaging the five senses is very important to making a setting real and three-dimensional for the reader, as well as for the character.
However, there are times where the scene is what is important, for impact, and not the character's interaction with it, times when the character is a mere observer:
Katon came over the ridge, relaxed in the saddle, unsuspecting. As he crested the rise, the view before him struck him like a hammer blow to the skull. He sat back hard. His horse stopped abruptly, first shying back, then rearing a few inches off the ground in response. Katon hardly noticed as he observed the valley in stunned silence.
Below the verdant acres of his childhood were replaced by tracks of nightmarish landscape. In the foreground, trees, shattered and blackened, loomed over land furrowed by violence where once it had only known the plow. In the mid-distance he could see what was left of the farmer's cottages, shattered like walnuts until only husks remained. Katon closed his eyes against further sight, but the tiny ridges of muted color scattered on the ground before those houses were etched in his memory.
Rough examples, but you get the idea. How much detail you provide and the way you provide it depend on the impact you need from the scene and the pacing of the story. If the point in the story you are working on is more about character development, always break things up with character-specific observations. Everything should be filtered through them as they are experiencing it. I'm not into setting a scene first and then engaging the character. A paragraph or two of straight descriptive always feels like an info-dump to me, kind of like a play script rather than fiction. "dressing" a scene before any characters come on the page slows things down and draws attention to the fact that things are staged, whereas experiencing the setting as the character does makes things more immediate, personal, and quick-paced. Now, I'm not saying there isn't a place for both, but pick and chose carefully/
To figure out what to add first determine what is important. What terrain will the character have to move around, what information does the character need to learn here. Think of the video game Myst. There are rich, lush landscapes, but only certain elements can be manipulated and have relevance. Others are just distractions or window-dressing. You spend more time on what is important either to advance the story or understand the character.
Now, for character description, that can be tricky. Some will say you can't describe a character in details they are not able to observe: for example when you are in the POV of Katon, it would be considered wrong by some to say: Katon looked up, his brown eyes smoldering until they were nearly black, and took a step toward the villain responsible for the devastation. See, since Katon cannot see what his eyes look like at that point in time, it is reasoned how can he know? It is information available only to the author and thus it is considered to draw the reader out of the character's POV and into the author's head, removing them from the story.
The opposing viewpoint is this is fiction, the reader knows they are reading fiction, so just write the character as you envision them even if the character themselves are not in a position to actually see what you are describing (ie there is no mirror or other reflective surface around) and move on with the story. Most readers will not even realize this is "wrong", and it is less contrived than trying to work in ways to convey the information such as having the character observe themselves in a mirror or giving their description when you are in the POV of a different character.
Really, it is what you are comfortable with. You just have to realize that some professionals you submit your work to might disagree and see it as a flaw in the story, and submit or revise accordingly.
Now, I write fantasy and science fiction and I find that character description is by far more important when I am writing fantasy. I just naturally add it in and incorporate it into what is going on, but with science fiction, or more specifically, military science fiction it seems unnatural to go into what people look like unless it is directly relevant to what is going on or will be significant in a future point in the story. I am told I am not alone in this, most military science fiction is slanted to a more military viewpoint, where the individual as an individual isn't the focus, but in relation to the team. There physical condition is more the focus, rather than physical description.
In either case, as with my scene description, unless I am making a point of something, character description comes in bits and pieces woven into the action of a scene, rather than all at once in a laundry list of points.