Try to get into the character someone you want to imitate. Inspired with them. Well, if you have an audio recording and even better live with this man. Listen to or view the recording a few times to memorize the tone of a man shades his tone, as he opens his mouth as he stands. To parody failed to copy some of the characteristic features of a person (in this case, the tone and style of speaking), also to achieve a comic effect.
When you listened to the whole record, turn each sentence separately and try to repeat it. Here, you'll need attentiveness, and imagination. Imagine that you are that person. If you succeed, the task becomes much easier. Copy the audio in the player and listen to it in transport. Of course, you may soon get bored, but the voice of the parodied will be for you an almost "family" and, at least, tone you will be able to mimic quite easily after a while.
If you have the ability and the desire then visit vocal classes or training courses on the formulation of the question. Only a man with a modulated voice and proper articulation can truly possess your speech, speak it low, then high, to use the richness of intonations. The voice, its strength and flexibility, can be developed.
Watch performances of well-known parodists, read their interviews. You may be able to learn some of their secrets and techniques.