How to make a script slow down to a stop

How would I make this rotating script slow down to a stop? Many Thanks Charles

 using UnityEngine;
  using System.Collections;
  
  public class SpinLogic : MonoBehaviour {
  
      float f_lastX = 0.0f;
      float f_difX = 0.5f;
      float f_steps = 0.0f;
      int i_direction = 1;
  
      // Use this for initialization
      void Start () 
      {
          
      }
      
      // Update is called once per frame
      void Update () 
      {
          if (Input.GetMouseButtonDown(0))
          {
              f_difX = 0.0f;
          }
          else if (Input.GetMouseButton(0))
          {
              f_difX = Mathf.Abs(f_lastX - Input.GetAxis ("Mouse X"));
  
              if (f_lastX < Input.GetAxis ("Mouse X"))
              {
                  i_direction = -1;
                  transform.Rotate(Vector3.up, -f_difX);
              }
  
              if (f_lastX > Input.GetAxis ("Mouse X"))
              {
                  i_direction = 1;
                  transform.Rotate(Vector3.up, f_difX);
              }
  
              f_lastX = -Input.GetAxis ("Mouse X");
          }
          else 
          {
              if (f_difX > 0.5f) f_difX -= 0.05f;
              if (f_difX < 0.5f) f_difX += 0.05f;
  
              transform.Rotate(Vector3.up, f_difX * i_direction);
          }
      }
  }