How does a writer keep a large number of characters distinct and accurate?

I got a question from a very eloquent adult writer who wanted to know how to keep her many characters straight as she wrote her book. My answer: this is a monstrous problem, especially if you're writing a series. I often copy ads off the Internet that look somewhat like the character they represent. For Shadow Souls, I even carefully picked out real clothes from expensive prom offerings so that I could visualize the dresses on the girls more clearly. I may post them on my website. Of course t...