Mentoring. A formal process for partnering newer workers with veterans is perhaps the best way of capturing what the pros call "tacit knowledge"—the institutional memory and tricks of the trade that typically don't get written down.
Such mentoring relationships don't have to end with the mentor's retirement; in fact, they're often easier to manage when both partners aren't juggling day-to-day tasks. Consider options for a phased transition where outgoing veterans' primary responsibility becomes giving advice and counsel to younger successors. And don't limit yourself to one-on-one pairings.
"Action learning." This is General Electric's term for pairing up rising stars with veterans on cross-function project teams. In addition to putting a variety of skills and resources to bear against important challenges, the GE strategy exposes young workers to important in-house knowledge while at the same time giving those up-and-comers a reason to soak up as much of that wisdom as they can.
It's an important principle to keep in mind: The best time to transfer your veterans' tacit knowledge to their successors is when that knowledge can be put to rapid and productive use. The action-learning concept is also a potential framework for setting up phased-retirement arrangements with your veterans.