Someone asked me this questions the other day and here was my response.
This is in regards to digital, but some of it applies to tradition as well:
The KPI's will always guide the work, so it depends on the project and what the sales/account management teams have worked out with the client, but some things that I hold in high regard are:
-How many placements? (how many published articles)
-How many links (only count if they are "followed")
-Did the placement of content result in a positive brand mention?
-Has my work strengthened our clients confidence in our companies ability to complete the outlined campaign? Difficult and time sensitive projects?
-How much added value did we provide on top of the requirements for the completion of the campaign?
It's not much, but it doesn't have to be complicated. In fact, it shouldn't be. Need some icing on the cake? Dig into your Google analytics and find a way to use the growth you've achieved to paint a beautiful picture for you client. Perception is just as important as reality. Especially doing digital PR.
If you wanted to get more detailed, you could even look at the acquisition cost of a new users and pitch the client on the overall value you've provided.
The hard work and hustle that you put into building relationships with publishers will only pay off if your able to effectively communicate (to your clients) the value that you've provided.