How To Get Higher Clickthrough Rates

I have been testing various techniques to get higher clickthrough rates for my newsletter, meaning "how do I get more subscribers to actually open the newsletter and read it?" The newsletter is about Internet Marketing, and all my subscribers should be interested in the topic, because that's why they all subscribed in the first place. My newsletter is free, and without any "hidden" offers, I am only giving away Internet Marketing Tips. So, I am thinking that it should be interesting to all my subscribers (100% of them) to open every single edition to read what I am up to this week. But, I am only getting about 15% of them to click. I tried to use a headline like "Internet Marketing Tips newsletter #1" and a title like "How to find the perfect Internet marketing writer". The second one being related to one of the topics inside the newsletter. So far, I have found the second one to get the most clicks. But it all depens on the topic, if it's an interesting enough topic, a lot of people will be clicking. I was hoping that it would be way easier to get people to click, but I guess that it might take time to convince my subscribers to start reading the newsletter. My first edition did only have 7% ctr (clickthrough rates), the one I wrote this friday have 15% so far. I guess I am improving. But is it all about content? If my subscribers find the content to be really useful, they will read every single edition? If they haven't read it before, they will someday, if I just get the subject to be interesting enough for them (and I will).