It depends what type of diet you are following and what type of carbohydrate manipulation. One method is cutting carbs and drinking large amounts of water (like 4 litres plus) a day and then carbing back up approx three days or more before the contest, whilst reducing water intake. Using diuretics is an option, but potassium sparring ones.
Natural diuretics are Dandelion, Licorice and boldo. However there are several methods of losing water, carbing up, dialling in etc. There is a large thread on here pertaining to just this, you'd have to search for it, I read through it about a month ago.