I know it is a convention -- at least in Unitarian Universalist circles -- for preachers to end a sermon with amen and sometimes other codas, like blessed be or shanti. This is about amen, if not the others. Stop it. Stop it now. Carl Scovel (boy, I wish he blogged!) broke me of the habit and I thank him for it.
Amen is the word of the people: a liturgical affirmation of what has been said. I imagine the practice is devolution of the prompt, "And let the people say, 'Amen'." (Which the people can decline!)
You wouldn't end a sermon "and we agree with you" so don't end your sermon (or newsletter column, blog post or what have you) with amen.