You need to gain an understanding of what the benefits of email marketing are. That way you can present those ideas to your customers. Try reading more blogs that focus on Email Marketing - like The Email Wars - http://theemailwars.com/ - (there are many more) these will help you understand how to position your offering.
Instead of asking clients if they want to do a newsletter, ask them if they want to generate business. The focus must be on the benefit. The newsletter is one important way to stay connected and inform customers about your latest activities, upcoming events, etc, It keeps your company top of mind and will result in additional sales.
You have a very good question. I think that what might help would be to identify with people who you know that do work that supports what you do and partner with them to write a story every other month, have them share their mailing list with you and now you have shared information. I recently did this with a friend who provides a service that could be picked up by the people I provide services for. This is a win for her and a win for me because now I have expanded my email list and perspective client list. I hope this helps.