Could you model your PR business to take equity in exchange for your services? Most lean startups wouldn't be able to pay you much cash, but I think a lot of them would be willing to give you equity. In fact, I think taking equity rather than cash would offset the "the results are difficult to measure" issue you mentioned; if I give you $5,000 to do some PR for my startup, then (to my mind) you have the money regardless, and you could be spending 60 hours a week for two months traveling the world and evangelizing about my startup to all the right people, or you could just ask a friend to post it to his blog and call it good.

If you have equity, however, then your profit is tied directly to the success of my startup, so I could rest assured that you would have a (literal) vested interest in making my company successful.