Describing my characters is something I've always struggled with. I guess that's why I write first person so much, there's less emphasis on describing your MC, less opportunity for it.
So what is the best way to do it?
Do you stop your narrative to drop a character bio in? Well no, you should try and give the reader your character description info over the course of the story as it fits in. However, you still need that short info-dump when you first meet the character, but you don't want it to sound too much like an info-dump.
Different authors do this in different ways, for example Laurel Hamilton, introduces all of her characters with hair colour, eye colour, height (Or at least she dose in all the books of her's I've read). Obviously there's nothing wrong with this, it works fine for her. But it's not going to work for everyone.
One article I red suggested that less is more when it comes to describing your characters. Giving them depth and dimension without too many words. (See the On Description post)
This is a character description that I worked up while doing the prep work for my NaNoWriMo story (more on that through November) trying to use the less is more principle.
By all the stars in all the heavens, she could be irritating at times. And why did she have to call him sir? He guessed it was her military roots showing. Not that you couldn’t already see it, in her bearing, her physique, the way she dressed. He was sure that if you cut her in half she’d have ‘Property of the SDF’ printed down the middle.