I've been writing an online newsletter for a professional services client for a number of years now and it has served them very well as a brand awareness tool.
As a specialist in professional services PR, I help clients understand the need to identify with the issues their own customers are facing. In this case it's tax and we offer a wide range of free, practical advice that would otherwise only be available to the very wealthy. Consistently, their newsletter, sent electronically each month, gets well above average open and click through rates and I put that down to these things. Try applying these guidelines to your own professional services pr activities, or newsletter copywriting.
Write about things that are going to make or save money.
Strike a balance between overt and implied selling - newsletters should be educational and informative, any selling messages are incidental.
Avoid overt biase towards your/ your client's services.
Regard newsletters as impartial sources of free advice and a medium for building a trusted relationship.
Disseminate each issue as widely as possible, to existing opted in subsribers and also via social media channels.
Form partnerships with relevant media who might wish to publish extracts of your articles, for one client, this is a tie up with a professional photography magazine - they're all small business owners needing tax advice after all.
Look for ways to guest blog extracts of articles on relevant community sites - again another good new audience source.
Above all, be objective and don't be afraid to experiment a little.
Ask yourself if you would spend time reading what you have written. If that answer is no, then think about what might be more interesting and adapt the content of your newsletter accordingly.