Did you set up a Facebook page?
Do you have a website that targets the customer you are interested in reaching?
Have you considered volunteering to get your name out there and to obtain testimonials? (this was a great opportunity for me that turned into more clients than I could handle at that time.)
You can follow up with those with whom you sent a letter and set up a time to meet them (assuming they are local). You can stop by with your cards and your brochures and maybe a small gift with your company name on it.
Try to capture email addresses and start a newsletter or send out a free tips sheet and then ask people to share this with others.
Consider establishing a referral program and reward people for helping you find clients.
And don't forget RFPs or Request for Proposals. They are a great way to get started to begin building your client base.