diet coke!

[Tamsin Lewis - "sportiedoc"]

Diet Coke/Coke can be an effective diuretic (i.e make you pee) - its down to the effect that the caffeine has on the kidneys.
If you are volume depleted (i.e dehydrated) then coke can still make you pee but the quantity of pee is gonna be alot less.  The answer is to drink hypotonic fluid (if you really want to go) - room temp/warm water is the best....  preferably drank whilst sitting by a dripping tap...  and then a coke (taken as medicine in this context Doc)   

[doc]

that diet coke sure as hell plays with my kidneys