Learn a new language! Be fearless when speaking it even when you know you're wrong. The results are humbling often hilarious and get you accustomed to frequent failure. This is also a good approach for people who are socially awkward or want practice just speaking to people. There are literally millions of people interested in doing "language exchange" sessions, where you do x minutes in your native tongue in exchange for x minutes in their native tongue. Not only will you learn a new language, but asking questions about idiom you do not understand is amazing practice for "anything" you don't understand.