If you are managing a team, a company, an organization and asking that question, you are missing the point.  We have been living the free agent lifestyle for about a decade now where people come and then go to the nearest greener pasture.  Gone are the days where employees will stick it out with their organization for 25 years.  Deal with it.

Of course, making sure that your team is paid well, equipped with the right tools and knowledge to do their tasks and given some flexibility to do the things they love to do - while these things make people stay, we already know that and I'm not going to talk about those.  What we need to do is to accept the fact that people jump ships at a faster rate now.  At this day and age, everyone agrees that it is safer to be on the move or at least ready to move.

The real question that managers or leaders of teams should be asking is, "How do I make sure my ship will not hit an iceberg if the captain decides to abandon the crew?"  Here are a few practical recommendations:

1. Don't put all your eggs in one basket.  We all know this yet we always succumb to developing that lone potential and making him into a superstar.  Some leaders have their own reasons for doing so, consciously or otherwise: "I see myself in him," "There's just not a lot of good ones out there." Yada, yada.  Look, you are taking a risk, and not a good one at that, if you are behaving like this.  Develop a succession plan where you have a number of back up people who can perform critical functions and keep things going if your main men are not there, temporarily or permanently.  Like basketball, a team is no good if it does not have a strong bench.
2. Keep your doors open for new people.  Everyday.  Even though your headcount is already filled, make sure you are always on the lookout for fresh talent and that you always have resumes "in the pipeline."  You never know when someone is just going to pack up and go to timbuktu.  Also, meeting new people everyday gives a strong message to your employee-base: Work your ass off, and more. Otherwise, you'll find your desk with a new guy sitting behind it.
3. Don't make/keep discussions about attrition a taboo.  The problem sometimes is that people don't talk about it to the right people and because of that they don't get too see all the perspectives when deciding to pack up and leave.  In most companies that I know of, employees don't talk about their intentions of leaving not until the last minute.  Keep your communication lines open to your team members so they they can openly talk to you when they are thinking about leaving.  That way, you are given the opportunity to give your thoughts and, if it's the sensical choice, save them from abandoning ship.  When they do talk to you about it, be sincere when having that discussion.  Help him make the right decision even if the decision eventually would be for him to leave.

Old-timers are not going to like this statement - Attrition is OK.  Really, it is.  Just as long as you can manage it.  And by manage, you are making sure that the attrition does not put a hole in your staffing for too long, it does not open the flood gates of more attrition and you are using this opportunity to introduce new and thicker blood in your talent pool.