Perhaps better, though, is to consider the companies you read about often that are in your industry, or are covered in places where you want to get coverage. Then find out which agencies work with these firms. Once you're down to two or three worthy contenders, ask for a client list, years of experience, types of projects the firm has handled, and portfolios and any press clippings. Make sure press releases are compelling and well-written. When used on an ongoing basis, firms typically work on a monthly retainer.
Quick tips Look for innovators.
Get your money's worth. Any PR professional worth paying for should be able to handle fairly sophisticated questions from the press. Ideally, they shouldn't serve merely as a funnel for requests to your office. If you retain an agency for a year or longer, schedule at least a yearly review. It's a great way to gauge their effectiveness and possibly freshen your perspective.