Selecting keywords is a bit of a science. You need to get into the minds of your prospective customers and figure out what phrases they'll type into search engines when they're looking for something like your site.
There are several services available at little or no charge that will help you research the best keyword phrases to use when optimizing your associate program web site. Look for phrases that are popular--but not too popular. There will be a little less competition if you select mid-grade phrases as opposed to ones at the top of the list. Make sure, however, that the ones you select still get lots of hits.