Gargle with salt water for several minutes every day. The water lubricates the throat, and the salt prevents the throat from getting sore and irritated. The gargling stresses the vocal chords and forces them to set.
Strain your vocal cords by talking loudly or screaming. Do this within measure, and do not strain your vocal cords too much. Otherwise, it can lead to irreparable damage.
Keep yourself hydrated with warm water. Dehydration can severely impact your vocal range and your ability. Drinking warm beverages will strengthen and loosen the voice box. The more powerful your vocal cords are, the more ability you will have in changing your tones.
Combine deepening your voice with slowing down your speech. By not speaking as fast, you can regulate the pitch in which you speak. Through careful regulation, you can discover which tones produce a raspy effect. After you have discovered the right tones, practice in those registers until they become more natural to you.