Learn from other people, let people learn from you. A great way to cultivate a more fulsome understanding of a topic is to try to describe it to others, to share your understanding with others. Mentoring is like writing that way, and like writing, you can't expect to be good at it without practice. I think sharing what you know with people, people who you believe are both less and more knowledgeable than you, is incredibly beneficial to any environment (work, play, whatever). MENTOR EVERYONE!
Couldn't agree more – when we started Thinkful (mentor-led training for front-end development), my co-founder and I mentored every student ourselves, and it was amazing how quickly it exposed every hole in our understanding of even basic concepts. Even though the team has grown and we now have 12 mentors, we still try to work with a few students because we enjoy it so much.