Ask HN: Key things to know before hiring a PR agency?

Our collaboration tool started getting traction in education, so I'm thinking of hiring professional PR for the first time to speed up adoption an attract more users. Getting good reviews by notable web sites had the most significant impact for bumping up active users in the past, but we've not been able to get any new significant reviews for a while, despite a lot of interesting development. I blame this mostly on me not knowing how to manage the relationship with journalists or not knowing how to package/sell the news to them, hence the idea to experiment with a professional service. Education as a market is particularly challenging as it's outside our area of expertise, so we lack contacts and knowing how to tell a good story, but seems to be an area of a lot of promising potential for growth according to our user stats. I've never hired a PR agency before, so I wouldn't know what to look for/what are the most common tricks and pitfalls. If you've had success using a PR agency before to generate buzz around your product, what are the key things to look for?