Share your code and collaborate openly with other programmers (if they can be great programmers, that's the best)

It's not only working with other people, but to review code, discuss different ways of doing things and learn for each other (even some peer programming). I've been in works when you worked with people, but each one takes care of their own code, since the design to the tests, and, well, you're on your own...

But when you can discuss freely what it's the better way of doing something and you make code together that's something that teach you a lot...