-Keep your email brief (three sentences, tops).
-Tell a story. Many of the stories on publications like TechCrunch are as much about the founders as they are about the startups. Look for an interesting angle related to yours and pitch it accordingly.
-Bloggers have to create several stories a week. Do as much of their job for them as you can by framing the outline of the story they will write in your pitch for coverage (related to previous point). If you started it as a side-project while in college, great, mention that. If your startup is in the same space as [hot trendy startup], great, mention that. Just make sure the whole thing is less than four sentences.
All that said- don't waste your time trying to get covered if you don't have a regular user base. Yes it is true, if you are covered it will result in an enormous spike in traffic. That spike will quickly diminish and you will be back to trying to find more users. I'm very happy for the author that BetaList turned into a profitable business, but getting covered and hatching a profitable, sustainable business because of it is the exception, not the norm.
Not to say PR isn't useful (and surprisingly easy to game), but that should come after you've grown a reliable customer base.