Whitefly is a major pest in the garden. One if the reasons why whiteflies are such a nuisance is because they spread diseases between and within gardens. Being sap suckers, they do however cause damage by themselves as well. They're particularly prevalent in the subtropics and tropics as well as in warm, humid weather though they can infest your garden at any time.
Ladybirds, hoverflies and predatory wasps all help control white fly so try planting some more plants to attract these - more sunflowers, buckwheat (great green manure), bergamot, yarrow, coriander, dill and white cosmos.
Whitefly is more commonly found on potassium deficient soils so if you grow comfrey you can use the leaves as mulch (or to make a tea) to improve soil potassium levels. Vetch is also a good annual ground cover for improving the availability of soil potassium.
Feverfew, wormwood and marigolds are plants you can use to help repel them.
A combination of all of these things should help you clear up any whitefly problems you might experience. Whilst these plants are establishing, you can also try a garlic spray to repel this pest.
Sticky traps can be purchased or you can make your own and these also offer relatively effective short term relief although they're more useful as an indicator for whitefly levels.