Whiteflies are difficult to eradicate. The early stage is called the larva stage and it usually is sessile, remaining attached to the plant and feeding through this early stage. While in the larval stage it covers itself with a layer of wax and the wax repels water-based sprays, often keeping the spray from contacting the actual insect. In addition, the larva usually attach to the undersides of leaves, making it much more difficult to spray a plant or tree thoroughly enough to contact the larvae. They also have a high potential for breeding large numbers very quickly, so missing any of them leads to rapid re-infestation. The adults look like tiny white moths, but in reality these insects are more closely related to scale insects and mealybugs in the "Homoptera" (now Hemiptera). The adults sometimes can be so numerous on shrubs that just walking past the shrub or brushing against it could result in a cloud of the insects flying around and getting inhaled. 

Yellow sticky traps placed near infested plants will attract a lot of the flying adults, and at the least will reduce their numbers a bit. This is unlikely to eradicate them and they will be replaced by new adults fairly quickly, but it does give relief. If you are able to spray the undersides of the leaves thoroughly you can have good results uses a horticultural oil such as Safe-T-Side or neem oil or even an insecticidal soap. These will not provide any lasting protection but can kill the larva present at the time. Another possibility is the use of a systemic like imidacloprid (Merit 75 WP) that can be applied to the soil below the infested plant. This could not be used on food plants like tomatoes but it is labeled for whiteflies on "trees, shrubs, flowers", so generally for many other landscape plants. If applied to the soil it may take a week or more to move up into the foliage where it then is ingested by the feeding larvae. If applied as a spray to the foliage it may affect them more quickly but not last as long. 

Monitoring the success can be complicated by the fact that the larvae and their waxy covers may persist on the plants long after they are dead, so monitoring for the presence of new adults is more reliable than expecting the infested leaves to suddenly be free and clear of the early stages.