shelleycat
If you don't have any stones, pain, frequent urination, dark urine, then you really don't have to flush the kidneys, you just have to help them do their job. All they really need is water! :) As long as you are urinating as much as you drink and it is clear, you're good to go. Drinking more water than usual IS a flush for the kidneys. It gives them a vehicle to get rid of all the stuff they have to get rid of.
The other thing that's nice to do for the kidneys every once in awhile is a diet that is very low-protein, no dairy, no spinach or lentils. This produces much less uric acid and phosphorous, so again, a vacation for the kidneys.
If you DO have stones and what-not, then you're in for a much different course of treatment. Drinking plenty of water and eating a good diet that keeps the body from being too acidic or calcium deprived is all you need to do to prevent stones!