Treat the person with respect from the time you hire him.
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For example. Be transparent with any equipment lists that document what equipment are in the employee's possession. Share the list at least yearly with the employee so there are no surprises (and the resulting badness) if an employee leaves. There is little else that generates ill feelings than an out of date equipment list for an employee (what do you mean I have to turn in that laptop? I turned it in two years ago. What!?!?! You want me to pay for it? ... etc., etc.).
Provide a great work environment so employees don't want to leave.
Look at what you think concerns you when an employee leaves, and then think about what you should do while the person is an active employee to prevent your concerns from occurring.
Don't solve the problem after it occurs, prevent it from occurring.