At the beginning of your upkeep, put a +1/+1 counter on enchanted creature.
When enchanted creature attacks or blocks, sacrifice Primal Cocoon.
8/15/2010 Primal Cocoon’s first triggered ability triggers at the beginning of the upkeep of Primal Cocoon’s controller (who is not necessarily the enchanted creature’s controller).
8/15/2010 The counters are put on the creature, not on Primal Cocoon. They’ll stay on the creature even after Primal Cocoon is sacrificed or otherwise stops enchanting that creature.