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Journalists are like anyone else: They wake up in the morning wanting to do their job well and write about stories that will be interesting to their audience. If you do your research and pitch them a story that's interesting and up their alley they'll appreciate it. If you spam them they'll ignore it.

Find reporters who cover your beat using http://muckrack.com (full disclosure: my company made that site), follow them on Twitter, see what they like to write about, and start building a relationship by tweets or email based on what they're interested in.

There's no standard for the email to journalists, but just keep them short and immediately say why your web app would be interesting to their readers.