The best way to detox and flush out your kidneys to keep them health is to drink plenty of water which helps you stay hydrated and also when you drink plenty of water the water helps to flush your kidneys of harmful toxins.
Drinking a beer or two every now and then is also a good way to help keep your kidneys healthy and even helps prevent kidney stones.
I used to get kidney stones and after I started drinking a beer at least twice per month I no longer had any kidney stones.
You can also buy some kidney and gallbladder cleanse supplements that help you keep your kidneys detoxed and healthy and can also help keep your gallbladder healthy.
Eating healthy foods such as fruits, vegetables etc and drinking plenty of water and other healthy drinks including orange juice, apple juice, pear juice etc is all good to help detox your kidneys and keep them healthy.