Well its not really that fun... But I would recommend drinking lemon juice, only breathing through your mouth and not drinking water.
If you scream and yell for a while you will lose it eventually, though it is quite unpleasant.
Go over use it.. Attend a game and shout at the bottom of you lungs while you have a very cold drink beside you!!=D
I'm loosing my voice and I want to more!
Trust me I lost my voice last night by shouting like someone insane, I drank, and smoked, and I want it back!!!