It's also often stated that there needs to be a reason for describing the character, because most people don't just randomly take stock of how they look. A very common (and in my opinion not very good) method is to have the character catch sight of him/herself in a mirror. Of course, if the character in question is very vain and that is part of the story, then the mirror trick would work, and would be appropriate.

My personal preference is to have a real reason for describing the character - and not a contrived situation, either. It makes it more difficult, to be sure, but it also makes it feel less like a description dump done 'just because.' But when you have finally gotten a situation where the description fits natually, how much should you put in? You could fill pages describing your character. A good rule of thumb I've heard is: no more than three items at a time.

When two people see each other for the first time in the story, choose three items that describe the character well. (I've been known to stretch it to four on occasion, but I try to keep it to two or three.) By varying your choice of attributes and the terms used to describe them, you can describe not only the person you are directly describing, but also can indirectly describe the observer.

Consider the following two paragraphs. Both are describing the same person, a lady who has just stepped up to a microphone at a conference.

1:

The MC stepped back from the microphone and smiled at the lady he had just introduced. She smiled back a little nervously, but her step was confident as she approached the podium. She tapped her notes on the podium to straighten them, her brown hair swinging forward to obscure her face as she glanced at them. "Thank you," she said, looking at the crowd as it slowly reclaimed its seats. Her hands gripped the back edge of the podium, and her left heel lifted out of her dress shoes as she shifted her weight to her right foot.

2:

The MC stepped back from the microphone as the lady he had just introduced walked across the stage, her long dress swishing with each step. She tapped her notes on the podium, and tilted her head down to address the microphone. "Thank you," she said, her rich voice carrying to every corner of the stadium.

In the first example, the three items were her nervousness, her brown hair, and her fancy outfit (implied by her dress shoes) as seen by the MC (the fact that he could see how she was standing behind the podium was a giveaway). In the second, they were her long dress, her height (implied by her leaning down to the microphone) and her voice as seen by somebody in the audience.

Hopefully you found that the amount of description was enough to get an idea of how she looks, but was not overbearing. A few pages later, when appropriate, another two or three items could be introduced - perhaps her eyes or her smile when she is shaking hands afterwards.

What do you notice first about a speaker who has just stepped up to the microphone? What does your significant other notice first? What does someone you know with different tastes, skills, or hobbies than you notice first? What do you notice first about yourself when you're the one stepping up to the microphone? How are they different, and what does that tell you about you?