• Have a project succession plan in place for key project team members.  This includes not only the project manager but key analysts, developers, and other hard-to-replace team members.
• Assess whether the project manager is already a flight risk because of prior circumstances.  For whatever reason, the project manager may be at flash point and ready to leave or already looking to leave.
• Document if there are there specific skills required of the position.  Some project management positions can be filled by a good generic project manager, while others may require a specific functional, technical, or industry skills. 

Next facet is actions if a flight occurs:

• Execute the succession plan if the planned successor is ready to assume control.  Sometimes the successor will be able to easily slip into the shoes of the prior project manager.  At other times the successor may have to share workload with his or her manager or other project team members to lighten the load and better minimize project impact.
• Don't expect much out of the fleeing project manager.  He or she may be physically be around for another week or two but mentally the transition to the new job has already begun. 
• Stay close to the situation.  Don't just assume that because there is a new name in the project manager box on the org chart that all is going to go well.  Keep close watch over the new project manager and be ready to assist if you see problems starting to crop up.
• Keep the team calm.  When a leader flees the situation is ripe for confusion, dissention, and infighting which can lead to budget and schedule problems.  Keep the team focused and motivated through the transition.  Most importantly, keep them informed on what is going on.
• Position the next successor.  This is good for two reasons:  it helps mitigate the risk of another PM flight and is a natural learning opportunity to build up new project managers.