The key to knowledge preservation is to build a community and retain a relationship. Here tacit transfer takes place as people get to know each other and share emergent insights, implicit knowledge is discovered through dialog and the meaning behind explicit information can be shared.
Knowledge comes in 3 flavors: tacit (what you know, but cannot tell), implicit (what you know but need help to remember) and explicit (exchanged via writing, stories or conversations).
People taking their knowledge with them and leaving 'holes' sure is a major problem, many companies now build in knowledge transfer as part of their outsourcing and contract agreements to see that knowledge from external parties is retained.
The 'hole' left when people depart the organization (or move to another group within the company) can be partially filled by ensuring relationships are maintained and there is an open communication channel e.g. a virtual community. A few companies have retirement clubs where former employees can meet and mentor new incumbents. Some organizations have 'work shadows' or paired work to foster tacit transfer and they have structured practices such as peer assists, AARs (after action reviews), retrospects and learning histories to ensure knowledge is captured before, during and after projects.
Retaining a relationship is the real key to knowledge preservation – no repository can subsitute for the trust and social capital or provide the context, empathy and learning that can be gained from an on-going relationship combined with the incentive to share. [Knowledge-at-work]