clamp rotation on rigidbody

I am making a snowboarding style game, the character is a rigidbody with a constant z movement, the turns are controlled by tranform.rotation. I am trying to clamp the rotation so the character can not turn more than 90 deg in either direction (effectively back up the hill..) I am using Mathf.Clamp but it isn't working and I can not get my sleep deprived head around it :s Would anyone be so kind as to help me out :) this is my code:

var maxClamp : float = 90;

var minClamp : float = -90;

    var myTurn = Input.GetAxis("Horizontal")*Time.deltaTime* turnSpeed;
    myTurn = Mathf.Clamp(myTurn, minClamp, maxClamp);
    var myStrafe = Input.GetAxis("Horizontal")*Time.deltaTime*strafeSpeed;
    myStrafe = Mathf.Clamp(myStrafe, minClamp, maxClamp);

thankyou :)

Have a look at the MouseLook script in Standard Assets...it does clamping like that and should be easily adapted.