Every few months—and especially now, after a long dry winter—give your houseplants a shower! Just plop them down right in the tub and turn the shower on, or if you’ve got outdoor space, give them a generous sprinkling with the hose. You can also put them out in the rain, as we learned from Laure's gramma. Just remember to get the undersides of the leaves too. And of course, let them dry before returning to their spots in the house.
Regular showering helps limit the possibility of insect infestation; it also allows water to penetrate the soil down to the roots, nourishing the plant more thoroughly than its usual drink and flushing out any fertilizer residue. Most plants do absorb some moisture from the air through their leaves, but not if the leaves are clogged with dust. You'll be amazed at how shiny and green your plants look once they're clean!
One caveat: Keep cacti and other succulents out of the shower to avoid over-watering them. Instead you can give them a sponge bath with a damp towel.