Engage people outside your normal circle. The Rule of Ten says that if you follow someone on Twitter who has more than 10x your followers, chances fade that they'll follow you and (more importantly) engage with you later. I focus on interesting people with less than 10x my number of followers, but who have engaging ideas.
If they value my ideas as well, then they spread them to a much wider audience than my own. A classic example is @urbanverse, or Cindy Frewen Wuellner. She's an architect from Kansas City; I first found her because she wrote a hugely inspiring post on what architects can do now that they're unemployed, to which I responded (joining the discussion, item 1.) It turned out that we have many (but not all) common interests.
The benefits of engaging with her have been numerous: (1) she has exposed me to several networks I didn't previously know about, such as #letsblogoff and #aiachat, (2) she has great ideas that I would not have addressed had I not engaged her, (3) when she values something I say, she promotes it to thousands of people I'd never reach otherwise. We don't agree on everything, but if we did, that'd be boring.
