You Can't. There is no permanent way of whitening. Teeth will naturally darken with time and eating and drinking certain food items. Crowns and veneers cover the teeth and can stay lighter. Bleaching solutions have to be repeated throughout ones life to maintain whiteness.
Zoom. Zoom whitening works pretty well however i would recommend using bleaching trays as a touch up.
Nothing is permanent. No tooth whitening would be permanent. Different tooth colors respond differently to whitening. Foods and drinks (especially tea, coffee, and colas) and smoking will re-color the teeth, even with routine brushing.