Turn-over in a School - How much is too much?

As we near the end of the school year and people start making decisions about what is best for them and their family, my mind always turns to the idea of turn-over. It is a well-established fact that teacher turn-over is high, but what percentage of turn-over is too high for a school to sustain positive growth?  We are a growing school and the influx of new blood is great, but losing talented teachers is a problem. What are your experiences at your schools?  What systems exist to help inculcate new staff to a specific school's philosophy?  Please comment below to help me out!