Kidney stones can be made up of different chemical deposits within the body. The most common is calcium oxalate, others are uric acid, cystine and struvite (magnesium, ammonium, phosphate). If you have had kidney stones once, there's a pretty high chance you can get them again. So it is recommended to make changes in your diet and nutrition intake. The first step is increasing fluid intake. But why should you drink more liquids? When your liquid intake is less, or insufficient, urine remains in the body and becomes concentrated and acidic, increasing the likelihood of kidney stones. But more fluids, means your urine gets flushed out and hence the risk of a kidney stone is reduced. So drinking lots of water helps. A lesser known and equally effective way is drinking lemon juice.

http://www.buzzle.com/articles/lemon-juice-for-kidney-stones.html