Washing and insecticide soap is about the only natural way to do it aside from sticky traps (which are better for monitoring and poor for eradicating white fly) and vacuuming (which can be hard because they tend to fly away at any plant disturbance).

Check for eggs (small, yellow cones) and larval nymphs (flat white/clear scale looking things) which you can remove before they become flies.

The problem with controlling white fly with any means is they're one of the most spazz'd out pests around...you could touch the bottom of a plant and white flies that are on top of the plant opposite of where you touch will scatter. It's hard to spray them when they scatter at the first disturbance to the plant.

Also, if you're going to use Malathion for indoor plants, you might want to apply it outside and leave outside for a few hours (or more) before bringing the plants back inside.

Those things are a gardeners nightmare.

I fought them for years on my indoor plants, and finally decided to just throw them all out one winter and start over the next spring. Glad I did so.

There's plenty of research on the little fellas, they have different stages of life that are very difficult to control, and have evolved to be resistant to many pesticides.

I don't know of any organic way to deal with them. I've had luck using systemic pesticides (Orthene) and a dormant oil spray. Both of which will seriously harm most indoor plants on their own, and you have to repeat two or three times to get them all.

White flies can be a problem, especially when a broad spectrum poison such as Malathion (which is not an organically acceptable product) is used because that kills the natural predators of the White Fly. Yellow Sticky traps can be used to trap the adult flies and insecticidal soap can be used to control the eggs and larva.
Perhaps this from the Universtiy of California might be of some help.

Here is a link that might be useful: White Fly

Elan, please don't ever use malathion inside. It's one of those chemicals that is very difficult to use safely. I do wonder about your use of the DE......since whiteflies hang out almost exclusively on the underside of the leaves in every stage of their development, how did you use it?

Please note that some plants are very sensitive to soap sprays.....do your research regarding that before using. Most plants tolerate horticultural oils very well ; there are some light weight options nicely suited for indoor use. Neem oil can be used quite successfully.

So what kind of plants are we talking about?

I'm curious about the kinds of whitefly predators that might be found inside.....

Neem will kill them according to the University of Florida and also a squirt of Dr. Bonner's soap in a spray bottle filled with water. Spray the top and especially the under side of the leaves thoroughly. Let it set for 1 hour. Remove it with the hose.

Neem did not work on white flies for me last year. I brought them inside in the fall on some Persian Shield I was wintering over and come seed starting time they began to multiply. I tried sticky yellow traps but it didnt put a dent in the population. When I planted my garden I began using neem oil once a week. The white fly population only increased. By the end of the summer I wanted to wear a respirator when I was picking tomatoes.

I left the persian shield and the lantana (which were also infested by the end of the summer) out through the first hard frost.

No white flies inside this year ! (The Persian Shield is slowly growing back from the bottom, the Lantanas may be goners though)