place it in Start() or Awake() function instead of Update. So you get something like:

var mySpecialVar;

function Start() {
    Debug.Log("Only once i want to show you my: " + mySpecialVar);
}

If you have to call it in function Update, if you can call it in the first frame, you could do something like

var debugged : boolean = false;

function Update ()
{
   if (!debugged)
   {
     Debug.Log("My Debug Output");
     debugged = true;
   }
}

(This would have the additional merit that you could just set debugged = true when you're finally building the thing, as Debug can be a performance-drag)

If you need it in a later frame, after some event, you'd have to fiddle a bit with where you set the debugged-variable, but basically this should do the trick...?

Greetz, Ky.