Handled steadily, business directories stop being a project and become a habit. Beyond that record the canonical version of the name, address and phone number somewhere the agency of record can find it, at least in a competitive market. In other words, more usefully, this is maintenance work, and maintenance work only fails when it stops.
Simply as important, set aside time to prune each quarter rather than treating it as a one-off project. The businesses that win here are not the loudest, only the most consistent. The work sits awkwardly between marketing and administration, which is why it gets skipped.