The best way is to be sure to have a few spare nights up your sleeve during your Iceland trip. A lot of the night tours for the northern lights have "guarantees" that offer you a trip the next night if you don't see the lights. The other way is to make your way to a darker location and spend a few nights committed to light hunting. Places like Vik offer decent accommodation, things to see and do in the area and are perfect for Aurora spotting.