Give away some kind of free software that is useful and production-quality. That something should also be related to a larger, growing trend. You'll generally get higher quality clients, a non-low rate, and a more fun project (since it's centered on using a tool that you built). And you also don't have to make cold calls--the software should stand on its own and send the leads to you.
I've learned this from my own experience writing/maintaining various open source libraries centered on using Parse with Ruby. The rapid growth of Parse has caused at least a handful of their thousands of developers to want to use it with Ruby (either for a Rails app, or a native iOS app with RubyMotion). Of those people, at least a handful have contacted me looking for a freelancer. Of those people, some have become paying clients.
I've learned this from my own experience writing/maintaining various open source libraries centered on using Parse with Ruby. The rapid growth of Parse has caused at least a handful of their thousands of developers to want to use it with Ruby (either for a Rails app, or a native iOS app with RubyMotion). Of those people, at least a handful have contacted me looking for a freelancer. Of those people, some have become paying clients.