Although formal English is most commonly used in writing, there are circumstances in which formal speech is also desirable. English becomes more formal when it is used correctly and in a more complex manner. Grammatical errors and careless word choices are common in spoken English. Fixing mistakes and using creative and precise vocabulary will lift your speech from casual to proper.
Be aware of the parts of speech. English speakers commonly replace adverbs with adjectives in casual speech, so "I'm well" becomes "I'm good" and "She runs quickly" becomes "She runs fast." To speak more formally, be precise in your use of the parts of speech.
Use correct grammar, even in phrases where the average person does not. Instead of "Who did you give it to?" ask "To whom did you give it?" Include the word "that" when introducing relative structures: "She believed that it was true" is more formal than "She believed it was true."
Choose precise vocabulary words, and avoid the word "got." Instead of "She got a new thing," say "She acquired/obtained/bought a new item." Instead of "It seems like it," say "It appears so." In general, words that derive from Latin roots are more formal than words that derive from Anglo-Saxon roots. For example, "pedestrian" is more formal than "walker."
Avoid idioms. In an idiom, the words do not mean the same thing together as they do individually. For example, "to kick the bucket" means "to die," "to come up with" means "to create or find" and "to take the plunge" means "to commit to something." Idioms are inherently informal and should not be used in formal speech.
Enunciate. Make a distinction between the letters "d" and "t," and include all the syllables in words like "difference" and "library."
Ending sentences with prepositions may be permissible, but if you avoid this your language will sound more formal. You can add still more formality to your speech by using the liquid "u." For example, pronounce "suit" as "syoot" or "news" as "nyooz."
Using the passive voice undeniably makes language more formal, but some people frown upon it as being too stilted or difficult to understand.