Carter Hart is on his way to being one of the best goalies in the NHL and he works on the mental side of the game more than anyone! Below are two great interviews with Hart. I highly recommend watching any video of him. His interviews are informative and his playing style is top-notch! Any goalie can learn a lot from him.
Near the end of the first video, he is asked what he recommends younger goalies really work on. His answer is skating!
FYI: Carter's personal goalie coach, not the flyers goalie coach, was my goalie coach growing up.