For large plant's how do I keep the dust off them??

Unfortunately, you can't keep dust off plants unless they are always covered. You can use one of those fluffy dusters with the extending wand, maybe with the help of a step ladder for really tall ones. Or, use a damp soft rag or sponge on each leaf. If you can lift them at all, a quick rince in the shower will work. Or, wait until a warm spring day and use mild preasure from the garden hose.