In most professions, results are rarely guaranteed. Doctors cannot guarantee the cancer will never return. Lawyers cannot guarantee an acquittal.
Publicists cannot guarantee publicity, or exposure, will sell books. Mr. Obie Joe knows that is heretical to say, especially among book people, many of whom ascribe publicity to a lottery ticket approach, and wait for that perfect media exposure to make millions of sales.
A more practical approach, and one that looks at a book sale as part of building a brand, a career and an idea, is to think about what kind of publicist works best for your goals:
1) Open ended publicist: You'll work with this publicist on retainer, with a contract duration usually lasting 3-6 months. Your goal is to build a brand recognition. Mr. Obie Joe can think of a few authors who benefit from this type of long-term relationship: 1) an author with several books in store, now or later; 2) an author with zeal for live appearances, with a book on spirituality, business, parenting come to mind; 3) an author very new to the game, and needs to sort out the options for both marketing and promotion of the book. Depending on the scope of exposure planned, retainers can range from $1-$5,000 per month (not in New York, though). Define goals -- i.e., how many bookings for media or live -- but also be flexible to change goals as other opportunities arise. This type of arrangement requires a lot of collaboration between author and publicist; authors willing to be humble, hardworking, and relentless most welcome in this arrangement. An author can expect results, but know the results are sometimes not as immediate as hoped.
2) Project based publicist: This type of arrangement might give you the most direct shot at tracking results. An author picks a very specific goal, and hires the publicist most experienced at arranging that goal. There are publicists who do only one task, and do it very well. Examples include generating/sending a press release; setting up a radio tour; or researching professional development venues. Success is easy to measure, of course, but only in the short term.
Ms. Obie Joe's recommendation is to interview both types of publicist. Present your goals, your book, and your background, and see if the publicist thinks she will be a good fit to your project. Make sure -- and this is a very important step -- that the publicist you do choose agree on expectations before any task is done.
That way, you will be the Good Author, and your publicist will be the Good Publicist.