Why do you hold water?? Its from an electrolyte imbalance (Na+, K+, Mg2+, Ca2+, Cl-, PO43-, HCO3-), with Na+ being the most important contributor to Fluid levels.
When you take in excess electrolytes, such as Na+, it results in a buildup of this ion in the extracellular space. In order to balance the Na+ concentration between the extracellular space and blood plasma, water diffuses by osmosis from the bloodstream to the extracellular space.
The reason why the imbalance occurs is because the kidneys can only maintain electrolyte balance to a certain degree.
If you want to lose water under these circumstances, then you must find a way for less Na+ to be reabsorbed by the kidneys. Thats what diuretics do!!
There are 3 types of diuretics, loop diuretics (ex. furosemide), Thiazides (ex. hydrochlorothiazide), and potassium sparing (ex. spironolactone). Use spironolactone periodically to keep fluid retention down!!
There you have it. A biology lesson for ya!
Later