Reducing your hydration levels before your photo's is very effective for the final look - but only if your body fat is low enough. If you are at 15% body fat and you dehydrate yourself then you're simply wasting your time. The theory behind this is that when your hydration levels are lowered, your skin appears to be "tighter" and thus exhibits more definition in your physique.
An effective way of doing this is to slowly increase your water intake. Eventually you would want to be aiming for consistently drinking 4-5L of water per day. Then, about 3 days out from your photo's increase this to 8-9L of water. The day before your photo, limit your water to about 3L (this is quite the struggle after conditioning your system to handle large amounts of water). Then, about 18 hours prior to your shoot, cease any water intake. Ice cubes or even sipping on small amounts of water will do.
A word of caution though, if you dehydrate yourself to any considerable extent then you will be placing a strain on your system - particularly your kidneys. I didn't go crazy with it...some people do. It's a personal choice though.