Be serious about commitment.
Have and enforce an anti-harassment policy. Make sure no one suffers retaliation or adverse job action as a result of making a complaint.
Have a policy which addresses relationships between management and non-management employees or between a supervisor and a direct report. For example, policies requiring that the relationship must be reported to the company are gaining popularity. These "confessional" policies re-emphasize the company's anti-harassment policy and get sign-off that the relationship is consensual.
Talk to me.
If you learn of a relationship that has ended, talk to the employees, confirm the relationship is over and that there will be no workplace problems as a result. Supervisors should make sure the employees know how to make a complaint, should anything go wrong.