Robert Scoble has posted a great answer to a question on Quora: "What is the etiquette to contact a tech journalist about your startup?"
You should contact a tech journalist only AFTER you do this:
Know and follow at least 50 (I have a list of 600+ tech journalists on Twitter at Tech News People).
Make sure your startup might be interesting to the journalist in question.
Know why said tech journalist MIGHT cover you and HOW he/she might cover you (IE, have an interesting story). If I was trying to get into the New York Times, for instance, I’d read at least 500 articles written there first and I’d even start putting them into buckets. Know what bucket yours will go in.
Know how your startup is better than other companies already on the field.
Keep it short.
Explain why you are an example of how the future is changing.
Bring up anything that backs up your story (for instance, do you have a famous developer who already changed the world at, say, Apple?)
Make it easy to dive in. Do you have a blog? A video? A bunch of people talking about you on Twitter or Google+ or Facebook? A successful Kickstarter or Indiegogo campaign?
Work hard on a great headline. Lots of us get 1,000 press releases a day. Think about how to stand out.
Get someone we trust to refer you. If Marc Andreessen emails me and says "Scoble, you gotta see this new company" you bet your bottom dollar I’ll stop and pay attention.
ALWAYS carry your demo on your phone or iPad and be willing to show in weird places. Accidental PR is the best. I’ve found many of my favorite companies away from "official PR."
Be persistent, but not overbearing. Sometimes trying a second try when you have something new to announce works. I don’t know why. It’s just the luck of the draw. The difference between being persistent and being overbearing is a very thin line, though. Be good at listening to feedback.
If in a crowded market (photo sharing services, for instance) you better be 10x better and you better be able to demonstrate that in 30 seconds or less. Google was the 17th search engine but in just one search I was hooked and saw why it was better.