Stay within reason, don't try too hard. Realize that there are some people who won't stay no matter what you do, try to identify those lost causes and focus your efforts on everybody else.
If those who are open to staying see people rewarded, in futile efforts to retain them, for being almost gone, they will eventually start leaving as well.
PS: and one more thing: retaining starts with hiring. If your hiring process strongly selects against those boring people who might actually stay longer than two years, then nothing will stop the perpetual churn except changing the way you hire.
If those who are open to staying see people rewarded, in futile efforts to retain them, for being almost gone, they will eventually start leaving as well.