Start with your current connections.
Attend the conferences of the industry you're in. Ideally, present/speak at them.
Blog about solutions for your industry if you can do it without coming across as spammy.
Keep an eye out for articles/blogs/etc on problems that your offering solves. Follow up with people directly about that and/or post to the blogs saying "we do this and here's why we're great". (If you do this, absolutely do disclose that you're self promoting.)
Find someone with industry experience and/or a rolodex. Pay them a decent commission.
Find the right search terms to drive people to your site and make sure you have a clear description of the problem the customer is facing, with simple glossy answers for how you can help them fix it.