If you have trouble keeping good employees, consider actually listening to the ones that you do have.
Seriously, if you have excellent employees, sit down with them and have a conversation with them about what they like and dislike about their jobs, and what you can do to keep them on your payroll.
Proving that you are serious about accepting feedback and keeping them around, as well as treating them like valuable members of your team, is how you keep employees.
If you need a good corporate-speak term for this call them “stay interviews.” That term will probably cause most employee’s eyes to glaze over, but it should make business consultants nod knowingly.