how would i cause the function to wait until it stops to function again?

I am trying to make a selecting script, but when i hold down mouse0 it cause the function to run continually, how would i cause it to only run once then wait till its done to run again?

this is what i have

 var hit : RaycastHit;
 var X = 0;
 var Been = 0;
 var hitName;
 function Update () {
     var ray : Ray = Camera.main.ScreenPointToRay(new Vector3(Screen.width/2,Screen.height/2,0));
     var Text = GetComponent(GUIText);
     if (Physics.Raycast(ray,hit, 2)) {
             if (Input.GetKey(KeyCode.Mouse0)) {
                 Debug.Log("You hit " + hit.collider.gameObject.tag);
                 hitName = GameObject.Find(hit.collider.gameObject.name);
                 if (hit.collider.gameObject.tag == "Tree") {
                     Text.text = "You hit tree!";
                     hitName.GetComponent(tree).use();
                 } else if (hit.collider.gameObject.tag == "Rock") {
                     Text.text = "You hit rock!";
                 } else if (hit.collider.gameObject.tag == "Ground") {
                     Text.text = "You hit ground!";
                 } else if (hit.collider.gameObject.tag == "Water") {
                     Text.text = "You hit water!";
                     hitName.GetComponent(water).use();
                 }
             }
     }
     if (Physics.Raycast(ray,hit,50)) {
         
     }
 }
 function StartOffTimer(Var,vartime) {
     if (Been == 0) {
         if (Var == 0) {
             Var = vartime;
             Been = 1;
         }
     }
     if (Var > 0) {
         Var = Var - 1;
     }
     if (Been == 1) {
     }
 }