Writing! Programmers need to be able to make themselves understood by other humans. You need grammar, style, structure, and lots and lots of practice. If you confess to your advisor or an English faculty member that you're not terribly interested in literature, but you want to do what it takes to become an effective non-fiction writer and communicator, they can probably point you in the right direction.

(And if your school should happen to offer a rhetoric class, take it/them.)