I set myself up for a daunting task. I have outlined a series of novels, set in ancient Egypt, covering a span of approximately fifteen years. Some characters will appear in every book, some will be recurring characters, though not in every book, and some may appear only once.
The task becomes: how to describe a character in each book without becoming repetitive. Since each book is a stand alone mystery, a reader could pick up any one of them and enjoy it on its own merits. But, too little description may leave a reader unsatisfied, too much may bore the series reader.
Since the characters will also age during the course of the series, it was apparent I needed to know as much about them as possible. I created a character sheet for each one: name, age, height, weight, hair and eye colour, etc. It can be very unpleasant trying to describe a character when you have forgotten their eye colour and have to look back through a 250 page manuscript.
But, more than just the basics, my character sheet allows me to add information from previous books. So I know when my character has a scar on his left shoulder from his last adventure. And, two books from now, it still better be there.
I know it seems like a lot of work, but my characters are important to me and the story. And, in order to make them important to the reader, I want each one to be different, yet remain fresh, through the entire series. Plus, the added benefit of having the information right at hand - it save so much time.