Conducting exit interviews is a good strategy in identifying the reasons why employees quit. You can then validate if these reasons are valid, and then work to improve your business in these areas, thus reducing turnover costs. Conducting exit interviews isn't always easy since those employees may not be willing to give you their time. Nevertheless, try to get them done all the same.
Why not apply to same strategy to those employees who have been with you a long time? Ask your long-term employees their top-3 reasons why they have stayed with you so long. Ask enough employees and you'll begin to see a pattern emerge. Summarize the top-3 reasons why employees stay with you and work very hard to improve your business in these three critical areas.