I've been building up a network over the last year. Facebook, Twitter, etc. When I went public with the release of my book I made a huge deal out of it and let people know that I was giving the book away for free. I also posted here.
Quite honestly the giveaway was pretty easy but build up the network took time.
If you want some really good advice on some of this check out Joe Konrath's blog. It's worth every word.