There's an old saying that employees don't leave a company, they leave a manager. No matter how much you like, respect, or appreciate an employee, if they don't know it, they may leave. Make sure your interactions with employees are always respectful, and that you look for ways to actively value their contribution.
You or the CEO may have a vision for where the company is going — but do all your employees? Some of the most successful companies are able to attract and retain great employees because they are great at communicating their vision all the way from the top down to the front-line workers. (And don't confuse vision with financials; it needs to be bigger than that, especially if the employee in question doesn't have stock options!)
Have you outlined a clear path for advancement for your best employees? Or are they looking at a dead-end job? They might be the best cashier / phone operator / junior undersecretary you've ever had, but that doesn't mean they want to stay in that position forever.
It pays to get to know your employee's goals and help them see how they can achieve them within your company.
No one wants to stick around in a workplace that doesn't treat them fairly. Do a quick gut-check: Are you paying women less than men? How about minorities?
Younger workers in particular, are not willing to stick around at a company that is racist, sexist, ageist, or otherwise discriminatory in any way.