Cash is the best incentive. It makes sence to pay board certified nurses more as their postgraduate education enables them to give more to their patients and to their employers
Public recognition is important. Board certified nurses should be identified, with photographs, in a prominent location clearly visible to patients and visitors. It makes the hospital look good, and increases patient confidence.
Board certified nurses should receive identification badges that include their certification (Jane Doe, RN, CCRN, for example). These badges should be given to them in a cerimonial way. Nothing fancy; just a presentation with as many of the hospital staff (including physicians and administrators) as possible. It can be short, simple, and take 2 minutes during shift report. The cerimonial context is psychologically powerful.
Consider giving board certified nurses a new title. For example, "Registered Nurse - Critical Care Specialist." This should also appear on their I.D.
Finally, offer nurses who achieve board certification reimbursement for classes, books, application fees, etc. that they had to pay for. Reimbursement should be contingent upon successful completion of the specialty requirements (if you don't pass, you don't get reimbursed). Also consider tuition assistance for continuing education activities required to maintain certification.