(I had another blog blunder where I accidentally deleted the blog I wrote. Oops. So this is gonna be an abbreviated blog.)
I believe that what Jesus means by "you are not to prepare your defense beforehand, for I myself shall give you a wisdom in speaking" is not that we shouldn't study the faith or reflect over what Jesus has done for us, but rather that our entire lives need to be a preparation of our defense. Jesus will speak through us when we are close to Him. He invites us to prepare our witness simply by being close to Him and getting to know Him intimately!