One other area not touched upon that makes code easier to read is:  "When you update code, make sure you do not leave code that is no longer relevent."  For example JT used:

   IF v_data_type_cd = 'STRING' THEN

      v_return_dt := NULL;

    ELSIF v_data_type_cd = 'DATE' THEN

      v_return_dt := v_value_dt;

    ELSIF v_data_type_cd = 'NUMBER' THEN

      v_return_dt := NULL;

    ELSE

      v_return_dt := NULL;

    END IF;  

instead of

  IF v_data_type_cd = 'DATE' THEN

      v_return_dt := v_value_dt;

    ELSE

      v_return_dt := NULL;

    END IF;  

If I had to guess, the original function (or its original intent) returned information about NUMBER and STRING data types.  But I cannot just "guess" when working on code so I need to double check the code to see if I am missing something, which wastes valuable time.