Drink three liters of fluids per day. Too much water can flush electrolytes from your system, so you should include some other liquids in that amount. Soups and juices (not too sugary) are suggested. Caffeinated beverages don't count!
Dr. Kyle (Mayo Clinic) says you should have three liters of urine output each day. No need to measure. :) Just make sure you get those fluids in! Water is best!
If you have kidney involvement, you must follow the advice of your nephrologist. Your fluid intake may be restricted.
Please also read the results of a study that suggests that any liquids are suitable to meet the requirements:
http://www.mmsupport.net/index.php?option=content&task=view&id=112&Itemid=2