Modular X causes an artifact creature to enter with X +1/+1 counters. When it dies, its controller may put its +1/+1 counters onto another target artifact creature, allowing those counters to survive the creature carrying them.
Arcbound Crusher can steadily accumulate counters as artifacts enter, while The Ozolith offers another place to preserve counters when creatures leave. Sacrifice outlets let modular decks deliberately move piles of counters exactly where they're needed.