Just came across Press Pass, a directory of journalists on Twitter categorized by media outlet, by beat and by region.
The Next Web has more background info, including an interview with the founders which includes their ideas for how they see the future of the site.
“You can also find out what [the journalists are] personally interested in, as Press Pass highlights the stories that they’re sharing through their Twitter feed. The site analyzes each journalist’s tweets, creating a profile based on that content – including what they’re reading, topics they’re interested in and who they’re talking to. Each journalist is ranked based on the number of followers they have and their number of tweets.”
The key here is to use this directory to build you Twitter lists of relevant journalists – you can start building relationships with them slowly, so that when you have some important news to share for which you want their help, you’ll already be known to them.
BONUS: See also MuckRack, which is a similar (but paid) service. It’s an aggregator, unlike Press Pass which is curated. You can of course also use various Twitter directories, but those will also pull in anyone who has journalism keywords in their bio.

Washington, DC 



[...] PR Departments Take Note: How to Find Relevant Journalists on Twitter. [...]