How do I train myself to not think about what I want to say?

How do I train myself to not think about what I want to say? I've found out that whenever I don't think about what I want to say and just say it, It comes out fluently. but it's so hard to not think about what I want to say. whenever I'm with friends I constantly thinking about what to say and than that causes me to have pauses and blocks. but when I don't think about it, it comes perfectly. How can I train myself to do this?