Listen to good English speakers such as Boris Johnson, Stephen Fry, etc. Listen to their speeches and copy the way they speak. For example, Boris Johnson knows how he can deliver a strong message and be heard with the use of specific, powerful, and colourful adjectives and adverbs that add strength, meaning, and urgency:
"profoundly optimistic"
"insidiously programmed"
"a deep human instinct"
"astounding achievement"
"deeply and dangerously wrong"
"vital task"
"overriding anxiety"
”colossal character”
Read classics. References to classic literature will add depth to your words and meanings.
Read poetry. Reciting poetry will immensely enrich your vocabulary. I recommend that you memorise new words, phrases and metaphors in the context, then try to use them in your speech. Aesthetics, colourful language, compactness of expression, and the use of metaphor make poetry a source of interesting expressions.
Remember, how you say it is even more important than what you say. Modulate your voice, stress key messages, and use pauses this will help you to achieve good educated speech. Fluent English Speech (British English) or Fluent American Speech (American English), and Power, Pitch, Pace, Pause.
I also recommend the app Business English Speech. It contains 43 lessons from the book Get Rid of your Accent for Business, Part Three, ISBN 9780955330025. It works best with an accompanying video course. This app was designed for top-level professionals, diplomats and lawyers. With this app, you will practice sophisticated English expressions and RP.