Hang out with people who are smarter than you are, so that you get lots of practice being the one who's wrong in an argument.

That could be easier said than done if you live in a fairly isolated environment. I would love to find a group of people who are all smarter than me and would want to hang out and debate various topics, but I have no clue where to go. My university doesn't seem to have very many people that I can chat with, and even the people at the graduate mathematics courses I took at one of the neighboring universities were not very stimulating for the most part. I think this is largely due to the quality of the schools, but there is little I can do about that (at least that I know of) for the time being beyond waiting for grad school and chatting online; this might be the only non-specialist community site where I find multiple people who consistently know more than I do.