When to restring a tennis racket (even if you haven't broken a string yet!)

You've been hitting with the same strings for months. The strings haven't broken but they do look kind of ragged. So when is it time to restring your tennis racket? What are the signs? What do you look for? How do you check the tension? When do you take the plunge and re-string the racket? Before cutting your strings, watch this!