Use a damp cloth on the top and underside of each rubber tree leaf to remove dust and light coatings of dirt and grime.
Add 1 teaspoon of liquid dish soap to a quart of water into a bowl. Mix well until the water is sudsy. Use a sponge or a soft cloth to wipe each leaf gently -- on top and underneath -- to remove any dried dirt. Use a clean damp cloth to remove any remaining soap residue.
Add a teaspoon of fresh-squeezed lemon juice -- or 1/2 teaspoon of reconstituted lemon juice -- to a pint of water in a bowl or other container and mix well. Apply the lemon-water mixture to rubber tree leaves that have dirt that cannot be removed with plain or soapy water. Wash the leaves underneath and on top. Use a clean, damp cloth to remove any lemon-water residue, which could cause the leaves to burn if placed in direct sunlight.