Public speaking, while the first few times are an absolute nightmare, is the best thing you can do to improve speech. It doesn't even have to be just you talking - I joined my high school debating club for the last 3 years and my ability to argue logically and formulate coherent sentences skyrocketed in that time. What also helps IMHO is reading old books (like Dickens) and stuff. Just getting familiar with understanding complicated sentences and learning new vocab is rly good for your verbal IQ.