I've just seen this posted on LinkedIn with the question.
"How can I increase the number of subscribers to my Newsletter?"
As Seth Godin recently said "the only good newsletter is one where people complain if they don't receive it".
I'm really sorry to tell you that nobody reads your newsletter, it will be about you and nobody is interested.
This is nearly the second decade of the 21st century, you are better off ditching the newsletter and putting the time and budget into something that your customers like and enjoy. Something where they enjoy being, it's friction less and natural.
Are you increasing your web traffic and you have more and more followers in social networks? If so, congratulations! You seem to be on the right track, but… Are you getting your visitors to buy your services or products? Are you increasing your revenue?