Actors inhabit a wide range of roles and the best actors can bring the most believability to those often incredibly different roles. One of the most important cues to the audience of character is the voice. To properly embody a wide range of characters, the actor must have a flexible voice. Compound this with the necessity to project vocal nuances in live performances so the entire audience can hear, and it becomes clear that voice training is one of an actor's most important practices.
Give up your bad habits. If you smoke cigarettes or drink excessively, you are going to need to give these things up. Both alcohol and smoke damage the vocal chords and smoking also interferes with your ability to breathe deeply and moderate breath control.
Do regular breathing exercises. Lay flat on your back and inhale deeply and slowly from your stomach. Exhale slowly, imagining that there is a candle in front of you that you can't blow out. Do this ten times in the morning and ten times before you go to sleep.
Practice diction. Clear, precise pronunciation is vital to being understood on stage. Work with a voice coach or practice with tapes to make sure you keep your consonants precise and your vowels distinguishable from one another.