How to Gradually Increase Speed with Lerp or Slerp?

Hey guys,

I'm wondering how I can have this code gradually build up to speed, instead of taking off like a rocket. I understand that lerp or slerp may be the case, but am unsure how to implement it into the code. If that's even the case.

         var relative : Vector3;
            relative = transform.TransformDirection(0,0,0.1);    
         controller.Move(relative * Time.deltaTime * forwardSpeed);

Any help or ideas would be really appreciated.

Thanks!