Speech therapy is
the best way to get rid of a "permanent" lisp or, at least, reduce it greatly.
Therapy that targets production of the sounds "s" & "z" at the beginning, middle & end of words differs depending on whether the lisp is frontal/interdental, lateral or palatal. A "whistling" lisp may be caused by dentures or dental appliances. See your dentist first for evaluation & appropriate referrals.