If one day I found myself running a big company where due to the sheer number of employees, a lot of people were leaving at any given time, I would institute this policy:
Employees would vest 5% of their options (or some other incentive) if they choose to leave at a time when their project has reached a stable state.
As far as I know, other than protecting their reputation, most employees have no incentive to leave at times that are “convenient” for their projects. The cost of someone leaving at a critical phase of a project (eg: just before launch) is probably more than the incentives for persuading them to stay on until a less critical time.
Hmm. One weird side effect of this is that it might encourage people to start a bunch of small, trivial, easy-to-get-into-a-stable-state-quickly projects so that they could vest faster.
This idea would no longer make sense if it requires a lot of oversight or complicated mechanisms to prevent abuse. Perhaps I should post this on halfbakery.com and see what other people come up with.