To speak well requires the ability to read well. Reading deep and wide allows the preacher to have more available at their fingertips. For an excellent start to understanding this aspect of the discipline of preaching, check out 'Reading for Preaching' by Cornelius Plantiga Jr.
In order to reach a large audience, the preacher must be able to rise above references that only a singular age, ethnic or cultural group can understand. Narrative preaching and the inclusion of stories allows the preacher to be relevant to people of various ages and backgrounds. Try listening to great storytellers and not just preachers. On Spotify, one can listen to Stuart McLean, Garrison Keillor and the great, Dr. Fred Craddock.
Listening to and reading sermons from the flowing authors will allow you to preach more imaginatively and affectionately.
Barbara Brown Taylor- various books on Amazon, some videos on Youtube
Tim Keller- Gospel in Life is a great resource for new and old sermons
Ellen F. Davis- her thoughtful and imagery are celebrated in the discipline of preaching, seen in her book, 'Getting Involved With God'