how can i change animation speed forever

i have an animation "idle" is very fast. i want to make it x1/2 speed. how can i do that. but im talking about "forever"

You can attach this script to any gameobject that has animations and change the speed to what you desire.

var animationSpeed:float;

function Update () {
    for (var state : AnimationState in animation) {
        state.speed = animationSpeed;
    }
}

Just put this in your Start. Nothing special about Start -- that's just the easiest place to put it. There isn't an Inspector setting for it:

animation["idle"].speed = 0.5f; // "f" is for C#

http://unity3d.com/support/documentation/Manual/Animation%20Scripting.html also has a pile of examples.