As Seth Godin says in this post today, there's an element of risk involved on the part of your client if they refer you to a friend (what if you screw up?) and some products and services, due to their nature, aren't easily referred in the first place.
Newsletters can however go some way towards boosting referrals. Here's some thoughts.
The fact that you are the publisher of a newsletter makes it easier for client to refer you. They won't be just referring a vendor but referring someone who is an expert in their field; someone who provides something of value (the newsletter) without asking for payment.
The newsletter itself can be a powerful referral mechanism. One thing you can do is put an up-front request in your newsletter for referrals. Say something like, "if you've enjoyed this newsletter, pass it on to a friend..." or "if you value our product/service, pass this newsletter onto a friend who might also benefit...".
The newsletter itself becomes a powerful calling card for your business, and if it's something of real value they might see themselves as doing their friends a favor by passing it along.