Reading aloud. I have a shelf full of plays and monologues. They are the closest you will get to natural speech. I used to read aloud every day -- minimum of 30 minutes. Now after 30 years, not so much. Still, I can tell the difference the next day. I also did simple breathing exercises for a long while. I do them now to put myself to sleep. Not sure if they help my speech, but it puts me in sleep mode.