As a leader, if you want to keep a team member, there must be advancement and development available. Although career advancement is largely a corporate influence issue, your actions as a leader can still have a significant impact. This includes career counseling, helping the team member network within the organization to increase his/her visibility, and assisting in the preparation for and pursuit of alternative positions within the organization.
Unfortunately, many organizations have no system for career movement; other times, team leaders block team members from moving because they don't want to lose top performers who are crucial to achieving the team's goals.
Again, realize the new emerging workers' mentality, your team members are increasingly looking at "what's next". If you are not partnering with them in that effort, you are more likely to fall victim to a sudden departure - not only from your team, but from the organization.
Although it may seem paradoxical that you would prepare someone for the next opportunity, this professional development is also what keeps them interested and engaged in their work and enhances their continued commitment.