To leach out soluble salts, remove and discard the white crust and some of the potting mix from the surface of the root ball. About one-half inch to two inches of potting mix can be taken off without harming the plant. Be careful not to damage the roots.
Remove the saucer and place the pot in a sink where water can drain freely. Using tepid tap water, rinse the potting mix by filling the pot to the top with water several times. Wait for the potting mix to drain well between rinses. Make sure the pot's drainage holes are not plugged. In extreme cases of excessive salts, rinse the soil six or eight times.
Scrape excess salts from the outside of the pot, the drainage holes and the saucer. Using warm soapy water, clean and rinse the saucer. Allow the saucer to dry.
Replace the potting mix taken from the surface with new mix and water lightly. Return the pot and its saucer to their original location in the home. Water the plant only when the original root ball feels dry to the touch.
Prevent soluble salt build up by reading and following label directions for foliage and indoor plant fertilizers. Also, try standing the pot on a layer of gravel. This allows the plant to drain well without re-absorbing water that has come out of the drainage holes, which carries high amounts of salts.
For more information, please see the PlantTalk script #1339, Leaching salts from potting mixes.