I build my list manually and when someone subscribers that means he or she or if a alien from outterspace is interested in the content I provide so I use that to promote products to them. I have about 10 pre-written emails and still adding more to my subscriber list and not all of them promote affiliate links. I don't want my list to get the hint I want to make money off each email. I provide tips, link to certain blog posts of mine and give out a Freebie once and awhile that I should charge money for.
Let your subscribers know that once and awhile they will get a freebie as people love free things and will subscribe to lists to someday get that one email with a freebie.
I am working on creating a new form to add on my blog.. Right now I have a subtribe to list button in the comment section but I want to create a form to explain what they will get if join my list..