502.3a - Flanking is a triggered ability that triggers during the declare blockers step. (See Rule 309, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn." [CompRules 2003/07/01]
502.3b - If a creature has multiple instances of flanking, each triggers separately. [CompRules 2003/07/01]
502.3.Ruling.1 - This may kill the blocker prior to damage being assigned in combat, but the attacker is still blocked. [D'Angelo 1998/02/03]
502.3.Ruling.2 - Flanking applies to blockers which are not assigned to the Flanking creature directly. For example, creatures that become blockers due to blocking a member of a band which includes the Flanking creature are subject to the Flanking -1/-1 ability. [D'Angelo 1999/05/01]
502.3.Ruling.3 - Gaining Flanking after blockers are declared will have no effect on the blockers because the time for Flanking to trigger has already passed. [D'Angelo 1999/05/01]
502.3.Ruling.4 - If an attacking creature has multiple instances of the Flanking ability, even one instance of Flanking on the blocking creature will negate the effect. [D'Angelo 1998/06/15]
Note - Also see Rule G6.6, "Flanking".