You don't need to be a great orator to change the volume of your voice.  Simply doing this often enough can make a world of difference when lecturing.  When you want to emphasize something you can really turn up the volume or you can lower your voice to a conspiratorial whisper.  Either can do the trick.  When you lower your voice to whisper you may want to preface it by saying that you don't want the students to tell anyone you told them this.