One option that may be interesting is that of amnesia - 
immortals not having any active memory of the years that 
they spend.  The memories are "stored" like files on a 
computer and only "opened" when they are needed.

	At SME we postulated that memories begin to blur after a while, so what
the remember is actually a sort of.. condensed version of what they lived.

-Berk' Watkins,
 A Member of Team SME,
 A Fragment of the Final Angel,
 -and-
 A Rabid Kyo Kusanagi Fan...