A referral program in place for current clients, as well as for businesses you network with that focus on your niche clientele. You need to know exactly what the program is, how clients an refer friends in, what if anything they’ll receive for the referral. Same with businesses. Business cards aren’t enough. A flier or presentation so they know what services you offer, what they’ll receive for the referral, that you will do the same (knowing our business numbers is helpful here so you can show the traffic they’ll get by partnering with you), how to refer the client to you, etc.
A seminar or class you can offer to a partnering business is also helpful and will expand your expertise and exposure for that companies clients.