Being in hydroponics, all I do is run clean water the last 3-5 days before harvest...if in soil, I'd simply stop feeding a week or two before harvest and water as usual. Things like phosphorous, once actually inside the buds, is near impossible to flush out. Like Nirvana said, if you haven't overfed routinely with your plants, you can get by just fine without doing any actual flush. Just water the plant clean water as she needs it. I think that flushing is only needed when there is an over-abundance of something in the media. We don't flush any other crop or plant in the world like conventional wisdom suggests that we should with cannabis.