The best way to get rid of water retention is to drink water as what happens is when you have excessive amounts of salt or other things that dehydrate you your body holds onto water so it doesnt dehydrate completely, to counter this you need to drink loads of water and your body very quickly realises it doesnt need to hold water any more and so releases it...
The negative side of this is you will be running to the toilet ever 20 mins but once that need to pee subsides you will know your water retention is gone.
Hope this helps
Paul