One of the best sources, brought up here every now and then is from Peldi at Balsamiq: http://blogs.balsamiq.com/peldi/2008/08/05/startup-marketing...

This was essential reading for me on a couple of launches.

As to Mashable in particular, they have a startup coverage series 'Spark Of Genius.' Was a dead simple pitch (it is a form) and coverage for me there really got the ball rolling.

For other bloggers find someone how covers your niche and pitch them --- never pitch the general email address of the publication.

And one last thing - it might be worth it to pick one primary target of the big tech news sites. Once one covers you they usually all jump to follow (ie weeks old emails suddenly returned requesting interview ASAP)