The Burned-Out Blogger's Guide to PR: http://www.amazon.com/The-Burned-Out-Bloggers-Guide-PR-ebook...
(I used to write for TechCrunch and have subsequently done PR consulting for startups. Feedback on the book has been quite positive; if you read it and wish it covered something, let me know!)
Jason Kincaid's book is really really good
I was the editor of Dr. Dobb's and am currently the editor of Java Magazine (from Oracle). I get hundreds of pitches a month. The triage greatly favors outreach about products in my site/magazine's wheelhouse and initiated by someone I know. The latter bit is key. If my contact is at a PR agency that I've worked with before, I know they know my coverage area and my interests. But mostly important, I know their client will be prepared.
This means that if I ask for a demo, the guy doing it knows how to do a demo. I also know the tech guy and the marketing guy on the call have a modicum of understanding about how the conversation goes. (For example, not saying "we were hoping to get some coverage because we just signed an ad contract with you guys.") And finally, I know the agency will not contact me unless the company truly has something to say.
Because of these factors, I know that there's a high likelihood that engaging with the start-up will result in useful content for my readers.
In contrast, dealing with an unknown company and unknown PR person, it's a crapshoot. I can spend a bunch of time and have nothing to show for it.
So, when I'm under the gun (which is most of the time), I will always favor the interactions that are most likely to result in content I can use.
I used to sell the book, but it's online for free http://austenallred.com/user-acquisition/book/chapter/press/
That being said, PR is spikey (traffic wise), also don't expect millions of dollars to pour in after you announce your product. The news cycle is quick and your can be yesterday's news in hours. The ones that stay relevant is what they do with that traffic after. Make sure you have systems in place to collect emails or track who your potential leads are.
PR is a better tool for branding your company, so when a VC wants to see how the public receives you or a customer is deciding to buy your product and wants to research if you're a legitimate company. "Oh! XYZ publication demoed the product and it looks legit."
Most important part in my mind. The effort that you put into helping a PR agency iterate on pitches will be the biggest difference between getting good results and being out a serious amount of time and money.