Keep the leaves clean by wiping with a damp cloth.
When a plant becomes crowded in the pot it can be split and divided. Gently remove from pot and loosen soil to reveal roots. You'll see they're divided into clumps, which can be easily seperated into sections using a sharp knife.
The sap of zz plant is toxic and, if ingested, could make you unwell so, after dividing the plant, make sure you wash hands well with soap and water.