You can break up long format control strings with ~ at the end of a line. For example:
* (format t "It was the best of times, ~ it was the worst of times.") It was the best of times, it was the worst of times.
The ~, newline, and all whitespace following the newline are removed from the output, so you can align the continued control string with the previous line.
The : and @ modifiers have additional meaning:
With a :, the newline is ignored, but any following whitespace is left in place. With an @, the newline is left in place, but any following whitespace is ignored.