Check what other items your referrals are buying. One great feature of the Amazon Associates program is that you get paid on items your referral buys, whether or not they buy the specific item you referred. Amazon stats will let you know what items you referrals bought. If you haven't yet promoted the item, and it's a fit for your blog or website, you should consider adding to your product list.
Affiliate networks offer you access to multiple (and by multiple, we mean hundreds if not thousands) of individual affiliate programs. They basically work by simplifying the signup process - instead of applying to become an affiliate for a each product you want to promote, you simply apply to join the network and instantly have access to hundreds of different affiliate offers.