Stop talking when you've made your point.
Begin and end every argument or talking point with the thing you most want the listener to note or remember. Early points tend to be remembered because they aren't yet competing with all the others to come. As one continues to speak, each new word or idea presented to the listener competes for memory space with all the previous words and ideas.
Items at the end of a talk tend to be better remembered because they are recent in memory, and are perhaps still in the working portion of one's memory. Additionally, listener interest is elevated at the conclusion because it is understood as the critical summing up of previous points made - and perhaps not listened to.