The best way to accomplish this clearing is to feed only water for about the last week (or less) of flowering. Some people add a dash of blackstrap molasses to the clearing water as they believe it helps add to the finished flavors. One teaspoon (5ml) per gallon is the usual dose.
If you have been growing the plant on organic nutrients then there is seldom a need to clear the plant since it has not taken up any foul tasting chemical nutrients. The extra N stored in the foliage will have been used up as part of the natural process of final budding.
Note: These days there are several products on the "grow store" market which say they help the plants clear chemicals. Botanicare Clearex is one of these which users report great results using. My take is, if its not broke why fix it. Good old water does the trick. :)
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