Two points. One: Weave your description in with the narration. Don't devote a paragraph to describing your character; mix it in with what's actually going on so that it's relevant. Remember, even if it's third-person, your story is essentially told from a viewpoint. The character's not going to sit there for a minute just thinking about what they look like and what they're wearing today. Even if you accomplish this by having them agonise over what to wear for a few minutes and then stare at themselves in the mirror all emo-like, wondering why the girl they have a crush on doesn't like them when they have such gorgeous brown eyes or whatever, it's better than just dropping it in there.
Two: As was said above, it's not always necessary to know what characters look like. I'm 21 chapters into Champion Game, and so far all the audience knows about my protagonist in terms of physical appearance is that he has somewhat unruly brown hair and he's not too tall for his age. That's enough to give you something of an image, but to be honest, it's not really that important. Is the character's appearance a plot point? Most likely not, so it might not be necessary to give it more than a passing mention.
On the whole, just don't make it too much of a focus. Tone it down, mix it in with your narration and don't let it steal the show. Be subtle. Even if it's 'Lucy shoved open the creaky old window, letting the morning breeze toy with her long blonde hair'.