Simple and competent speech - the key to success.Therefore, purify your speech word-parasites, jargon and other verbal garbage. It greatly cheapens the vocabulary.Try to speak without errors (correctly put the accents, coordinate the words in number, gender, case, etc.). Do not use unfamiliar words as well as words that may be confusing to others.
Constantly improve your speech. Watch talking TV presenters, journalists, philologists. Chat with people who are smarter and better educated than you. Often look in dictionaries and reference books, especially, if in doubt about the pronunciation of some words. You can read books on rhetoric (Sergey Shipunov "Charismatic speaker", Boris Timofeev "whether we're talking").
To make your voice sound more persuasive, work on vocal technique.To do this, do special exercises, learn tongue twisters, tell me with the expression of a poem, correct speech defects.