Resolve to help your clients help each other.
Step One: In addition to your normal engagement agreement, develop a “Client Promotion Agreement” that your clients sign that permits you to discuss with others what they do (in a most generic sense) and allows you to introduce them to others who can help them/buy from them/sell to them/etc.*
Step Two: When asking them to sign the Client Promotion Agreement, explain to them that you take their privacy very seriously, but also believe in helping them and their business in any way that you can, and that you have many clients whom they might benefit from being introduced to.
Step Three: Get to know as much as you can about your clients’ non-legal needs. Try not to charge for these conversations (and do it at their place of business, if you can). Ask them questions like these:
What are the most common problems your customers have that you aren’t able to help them with?
What one thing could you do this year with someone’s help that would have the greatest impact on your business?
Step Four: Introduce them to others who can help them.
* Though you may not ethically need this agreement (or you could cover it in your engagement agreement) it is a good way to reinforce how much you care about them and a nice way to begin the rest of the conversation about how to help them.