Colleges have a breadth requirement; one source I read suggested using that to take a writing heavy course in history or philosophy that requires lots of short papers in order to improve your writing.
It's also possible to get lots of writing experience by taking advanced foreign language courses.
To further elaborate the question: What are the most universally and most useful skills one could learn at a university? Currently, I am studying physics as an undergraduate and I am thinking about my career options. I have gotten opinions in other forums as well but I would really like to hear your thoughts and I also think that other LessWrongers would find such a list useful. To give you a rough idea of what I mean I have two examples:
What are some other skills along this line that are universally useful?