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.