Getting connected with some VARs (value added resellers) can be helpful. If you can provide some piece that enhances their package, you can often get included in as a part of some big deals that you might not otherwise have any shot at, and build up some connections that way. Once you're in, and people know you as somebody that does work in X niche, they can start coming to you to do additional customizations or follow-on work in that area.

It's not all roses; sometimes the extra layers of indirection between you and the actual customer can be a little onerous.