Does anyone know how to retain a good employee when s/he decides to leave?

Job Exit Interviews The best business policy for retaining employees is to avoid the letters of resignations.
Rather than repair damaged workplace relationships after an employee informs the company of the intention to leave, preventative action includes asking staff for feedback about job conditions during the year.
When an employee submits a resignation letter, however, an exit interview offers valuable information to stop other staff resignations.
A workplace manager or small-business owner who takes time to have an honest conversation with an employee after receiving a resignation notification helps identify reasons for the worker's departure.
http://smallbusiness.chron.com/stop-employees-resigning-33302.html

Actually mostly resigned employees very difficult to retain, such employees are already in exit mood and have decided to leave.
First, it is important to distinguish between two types of employees: the core and the peripheral workforce; the core employees are those who enjoy high flexibility and have various skills and good attitude to work; such employees are core because those whom you need to retain not the peripheral workforce whom on the other hand are the employees whom are of standard performance and in case of any market fluctuation you will be more tending to lay off from this force.
In case one of the core employees are submitting his resignation, to retain him it is necessary to understand the essential reasons behind his willing to leave.
Accordingly, you should have an offer to him that will meet his needs.

First of all have a meeting with that employees for assessment and knowing his/her problem why he/she is going to resign and motivate them to continue with you and giving him/her some motivation/reward.if he or she want any promotion or salary increment then i think discus with your top management and satisfy him/her that this thing will be added in annual evaluation.