Then set out to build relationships with them... start commenting on their blogs, retweeting their stuff, emailing them story ideas (don't go nuts, keep this short), linking to their posts from your blog (oh yeah, you should be blogging for your prospective user audience), commenting where they comment (see Backtype), you get the idea.
Once you feel like you've put in a couple months of solid two-way relationship-building, reach out and offer to "brief" them (i.e. give them a demo and talk to them about how you think about the market) and give them a beta account. Tell them when you're launching and that you'd like them to hold off from posting until that date. Do this with 10-20 influencers for 1-2 weeks before launch.
Here's some additional reading I'd recommend:
http://searchenginewatch.com/3640596 http://www.businessinsider.com/2008/8/jason-calacanis-on-how... http://www.bothsidesofthetable.com/2010/03/22/6-tips-to-buil...