[digikata]

If you want to reduce the "golden handcuff" effect, then you can keep an account of each employees 'kicker shares', but continue to issue shares on an x every time period basis. This causes inflation in the currency of 'kicker shares'. If you stay on continuously, then you keep your percentage of the kicker. If you leave, then those shares you earned slowly deflate in value.

You could even recognize higher risk of earlier employees by issuing special shares which have some mechanism by which if they leave, those shares may still deflate, but at a slower rate than later ones. e.g. for every time-period distribution of shares, these shares receive some fraction of the new distribution.