One of my former startups was covered in many publications (Techcrunch, USA Today, NY Timers, Venturebeat, etc.). We used multiple consultants and PR firms with varying results. I've found that if you have something very newsworthy like a product launch, consultants can use their relationships to amplify your coverage. Especially if the consultant/firm specializes in your industry.
However, I think of hired PR more as an amplifier. If you don't have newsworthy stories, it's hard for them to generate great press. The companies themselves typically need to generate these ideas, which can be a lot of work. It can be a lot of fun too, but you have to make sure it's a good use of time and focus versus everything else you could be doing.
I normally try to brainstorm along various categories of stories: product/feature launches, interesting stories based on data (OK Cupid and Pricenomics are great at this), customer success stories, guest posts by the founder, etc.
Finally, I think many great startup founders form real personal relationships with journalists. This isn't easy to do, but if you actually become friendly with writers they are much more likely to at least consider posting about your company. Good luck!