Pitching stories and potential guests to journalists is a key part of a PR's job.  Here is a great example from today's CityAM on how not to do it.  Everyone working in PR should take a few moments to read. 

PRs in professional services firms perhaps have it easier than those working for 'me too' tech companies, questionable consumer brands, and former Apprentice stars turned consultants. Journalists, more often or not, will welcome a well timed pitch offering insight or commentary. 

But the rules remain the same.  1. Do your homework. Read a title first before making a pitch. 2. Have something to say. 3. Build relationships with a few key journalists. 4. Email first. If its a really good pitch a journalist will respond.5. And if you really must chase, but choose your timing.