There are lots of answers, but two things will help. One, be a niche marketer. You can be a serial niche marketer, but always try to establish yourself as the expert in one niche at a time. For example, SEO and web design is one thing, but how many people bill themselves as the SEO and web design expert for jewelers? Or music shops? You can sell to everyone in a niche if you are convincing, and believe me they all know each other and will refer you to friends (both in and out of the initial niche, which might be the start of the next niche).
Two, more than broadcasting yourself, look for inside connections. Network with peers in a different industry, preferably your niche industry. (Remember that peers are decision makers, not secretaries or salespeople.) Do this by giving talks or presentations, not at a local hot-seat wham-bam-thank-you "networking" group (although every once in a while there are good networking groups, they usually have other functions like politics, charity, community, etc). Freely give information and suggestions. Show that you really are the expert. It will help to be able to mathematically describe your results to these peers (with real marketing research, in which you know exactly what caused the beneficial effect) so that you can really show them. You can do this in a few words or in a presentation to an industry group. When your cross-industry peers need someone with your expertise (or when you've convinced them of the many benefits), they'll be knocking down your doors and blowing up your phones.