This is a quick, and obvious one: Be prepared for the worst. What would you do if your two best employees quit tomorrow without notice? How long would they take to replace? Do you have people that can do the job? If they are management, do you have talent ready to promote?
Sometimes it means over-staffing. But sometimes, that's necessary to keep things running smoothly. Have contingency plans. Develop talent to be ready for promotion sooner rather than later. Be ready. Don't screw yourself over.