In addition to cleaning over 200 gallons of blood a day, the kidneys regulate blood pressure, aid the maturation of red blood cells and ensure proper functioning of your nervous system by keeping serum levels of potassium constant. They do not need to be "cleaned" to perform these functions. The only thing cleaned by such practices is your wallet.
Food and Kidneys
Most "kidney cleansing" schemes that involve food and herbs seem to be based on the mistaken idea that the kidneys are some kind of digestive organ. Unlike the stomach and intestines, food never sees the kidneys, so food could never clean them. Instead of thinking of the kidneys as part of the digestive system, they are more properly considered either as part of the circulatory system or as a system unto themselves.
Some people think eating watermelon cleans your kidneys because it contains a lot of water. This practice is downright dangerous for kidney patients on hemodialysis. These patients must limit their intake of liquids because of their compromised ability to urinate. Consuming more liquids than the nephrologist recommends creates serious problems because the patient cannot get rid of that liquid until his next dialysis session.
"Cleansing" schemes based on watermelon and other fruits are dangerous because these foods are often high in potassium. Many patients with advanced kidney disease are no longer able to regulate the amount of potassium in their blood. Many fruits, including watermelon, have more potassium than patients can safely eat. If potassium levels creep above 5.0 mEq/L, nerves begin to misfire, which causes irregular heartbeat. Heart attacks and death can result from this kind of "cleansing" scheme. Kidney cleansing schemes based on herbs often raise this problem as well because the potassium content of herbs is never considered.
Instead of cockamamie "cleansing" schemes, you can prolong your kidney function through common sense measures like lowering your blood pressure, if needed. Hypertension is very common among kidney patients because the kidneys are involved in blood pressure regulation. Taking your blood pressure medications, getting enough exercise and eating a low sodium diet can go a long way toward preventing kidney disease from getting worse.