Practice obviously but what is more subtle is where/how do you get the practice. A turning point for me was to realize that they are many moments of your daily life where you are next to someone and for some reason, you want to say something. More often than not, you dismiss this opportunity and keep to yourself but the issue is that these moments happen often and that over the years the compound effect is enormous. I actually made a short video about it, let me know what you think :)