When I used to write product reviews, I used to include just one affiliate link. For some reason, I thought that a single link would be enough and I didn’t want to run the risk of annoying readers with more links. However, one day it struck me that the reviews I was writing were quite long and by the time people got to the end of them, the link to Amazon was no longer visible.
You should also make sure you aren't competing with your own affiliates for eyeballs. Any marketing channels you're using, such as search engines, content sites or e-mail lists, should be off limits to your affiliates. Put marketing restrictions into your affiliate agreement and notify partners immediately. It's your program--you set the rules. Or, if you prefer, you can let your affiliates run the majority of your internet marketing.