I definitely agree that news-letters can convert very nicely. A data-base of interested visitors can be a very powerful thing.
Besides that, when thinking of content, try to work backwards: What are you trying to sell. How does everything you do and write bring you closer to that goal.
Not very romantic advice, but the basics work. That said, the very best thing is to ask your aff manager for specific advice on your specific site. Most of the time the problem is that your site is not optimized (see point 2), which is something that is hard to address in a theoretical or general way.
PM me if you want me to take a look at your site. :hattip: