Early on in nearly every story, the writer comes across the necessity of doing a physical description of their characters, and their main viewpoint character in particular. There are two basic schools of thought on this. The first is to keep details to a bare minimum – maybe just hair and eye color – and let the reader fill in the rest of the picture for themselves; the second is to go for a more detailed and complete description.
Since the first way obviously doesn’t take much doing, I’m going to talk mainly about the second. In my experience, readers want to have height, general age, hair color, and any instantly-striking unique aspect of the character’s physical appearance (wears an eye patch, has portwine birthmark over half of left cheek) as soon as they possibly can get them. Most of them are willing to wait at least a little for other things (but probably not all the way until the last half of the book), though some will still grumble. A few prefer to get a large lump of detailed physical description the moment a new character walks onstage, and with the right story and style, this can work fine. Usually, though, you’re better off working details in a few at a time over the course of the first few pages after the character appears, especially if he/she shows up in the middle of a conversation or a battle that you really don’t want to slow down.
Assuming that you know basic stuff like hair color/eye color/height/build/birthmarks/etc., the problem is getting them across to the reader. For non-viewpoint characters, this isn’t too difficult; you can just mention particular features as they come to the viewpoint character’s attention. This has the added advantage that the most striking features, like unusual height, strange hair/eye color, visible scars, etc., are likely to be the things the viewpoint character notices first.
The real problem for a lot of writers comes in describing the viewpoint character. If you’re doing the kind of viewpoint that zooms in on the character, you may be able to get away with a straight descriptive paragraph right before the viewpoint settles into tight-third-person, but that tends to be most effective if there’s nothing else interesting going on. If you’re zooming in on a character in mid-battle, or even just one in the middle of painting the living room, the block-o’-description tends to be more like a stumbling block than anything else.
That leaves working the description in as you go along. That means things like implying the character’s height by what he can reach or what she has to duck under. You can get hair length and color out of having it fall in the character’s eyes, or by way of comparison to other people (“his hair was about two shades darker than mine, but with streaks of pale blond lightening the brown”), or by the kind of adaptation he/she has to make (or doesn’t) to circumstances (pulling very long hair back into braids for a workout or to keep it out of the cookie dough, for instance). Clothing styles and fabrics can come out in the same way – most people who can afford to wear silk will be either worried or annoyed by the likelihood of having it ruined by rain or a spilled drink, for instance. And if they’re not wearing silk, they can be glad they’re in their gardening gear and not wearing anything good when the car splashes mud all over them.
You can also sometimes get other characters to provide descriptive details, ranging from a squeal of “Darling! You’ve grown six inches since I saw you last…and when did you grow that beard?” to sneers about old clothes or glasses to evidently well-meant comments about fashion sense, inappropriate dress, haircuts, and so on from parents, siblings, significant others, or plain old busybodies. Introducing such a character purely to provide description is not generally advisable, but if you happen to have one already to hand, by all means make use of them.
When you’re working with a first-person or tight-third-person viewpoint character, their self-description can become as much about their self-image as it is about what they look like. This is especially true if you’re describing your viewpoint character through comparisons with other characters (whether those others are present or whether they’re just memories). You can get some really interesting mileage out of having the character’s internal self-image not jibe with the way other people see them. It can be quite a jolt when the character who has been thinking of herself as grotesquely overweight, for ten chapters is described by someone else as “solid” or “stocky and well-muscled.” You can’t, however, get away with having a character think of herself as blonde for ten chapters and then have someone else describe her ebony hair — not unless there’s been a dye job in there somewhere.