#1 Create a list with all possible keywords that fit to your product, service or whatever you are promoting. The more words you come up with, the more chances you get of finding the niche keyword which brings me to point 2

#2 Niche Keywords. Drill down deep into the word, expand on it. For example, if you are promoting affiliate products related to mothballs. Instead of just mothballs you can use mothballs for sale, mothballs for sale in uk, mothballs on the cheap , discount mothball etc