(Exercises re-published with permission from VoiceOverVoiceActor.com)
Flip channels on your television or radio for 5 minutes to listen to commercials. Each time you find a commercial, try parroting the voice actor who is speaking. Try to repeat not only the words, but the musicality, the nuances, the tone, and the inflections.
Then turn off the radio or TV and pick up a random piece of text (it can be an advertisement in a magazine, a book, a piece of mail, etc). Try to use the same vocal patterns, tonality and style you were just mimicking as you read this new material.
Practicing this will begin to train your ear, attune you to what is currently "hot" in the advertising world, and get you reading and speaking aloud, which is important in and of itself.