I asked that same question to a panel of journalists from Wired, the San Jose Mercury News and VentureBeat. You can watch their point of view on how to get press at http://thefounderconference.com/video-archives/
Now, for my opinion.
The #1 reason why you don't get covered is that you are not news. Your story is boring and uninteresting, at least to that particular journalist.
In practice, here's a tip I use: until you can tell me the difference between two writers who work for the same media, don't bother contacting anyone. What do I mean? You will need to build some kind of relationship with a specific writer. Which writer is that? Don't spam "email@example.com", it's worthless.
Make sure you understand what your target writes about. Some blogs focus on fundraising: if you didn't get VC funding or money from a famous angel, it doesn't matter that a startup with the same technology just got covered. Other writers focus on life stories and would never write about raising money.