You can also try Neem oil. Or, best yet I've found, secure the soil by wrapping tightly in cheesecloth or gauze. Take the plant outside, hold it upside down, and spray the bejeezus out of the leaves and trunk with a "houseplant safe" insecticide. For me, soap and Neem oil have never worked 100%. But then I also have a largish collection packed fairly closely together.