I consider this my most important role at my startup, which is a self-service, two-sided marketplace modeled after Match.com for facilitating product donations between companies and charities. From my B2B experience I would suggest:
• Professional associations: Find out where they get information to understand their priorities and challenges better
• Industry events: Meet them in casual, social settings
• Agencies and fellow service providers: Partnering with others that serve the same community, especially if your platform makes their jobs easier
• Testimonials: If you wow one or more well-known or influential clients in a specific industry, others who trust their advice and judgment will follow.

Lastly, I'd suggest networking, being generous with sharing your knowledge, and becoming a trusted expert in your field. You want to be the one someone thinks of when they, or someone they know, is seeking a solution to the problem you solve.