Surround yourself with British people. Plan a trip to England and spend time interacting with the natives. The fastest way to learn to speak with an English accent is to immerse yourself in the culture. Natural changes in accents develop over time and are influenced by geographic location, culture and personal needs. If this isn't an option for you, meet British friends locally. Browse social networking websites and frequent British-owned restaurants and pubs to make connections with local English natives.
Watch British television for exposure. The BBC, or British Broadcasting Corporation, is widely available through many cable providers. Watch the BBC for at least an hour each day to continually expose yourself to the English accent. Pay close attention to the way words are pronounced and the way certain syllables are emphasized. Notice colloquial and slang phrases that catch your attention. When you learn to use them correctly, you sound more authentic.
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mat.usc.edu/TESOLPractice with a book on tape. Many books on tape are read with an English accent. Purchase one from your local bookstore. Play a few paragraphs of the book and listen to the English accent closely. Rewind the tape and start it again. This time, read aloud with the English accented voice on the tape. Try to imitate the voice's pronunciation and intonation as you read. Reread the paragraphs along with the tape until you have mastered reading them in an English accent. Do this exercise daily until you complete the book.
Use the Cambridge English Pronouncing Dictionary as a reference. This dictionary provides the phonetic pronunciation of words for both British English and American English. If you are unsure of how to pronounce a particular word with an English accent, look the word up in this dictionary. Use the British English pronunciation to say it correctly.
Record yourself speaking with an English accent. Use a cassette recorder and tape. Play back what you recorded to identify which words you still need to work on. Practice the words you identify and rerecord yourself. Do this exercise at least once a week to monitor your progress and improvement.
Dynamic Speech Co. American Accent Pronunciation