Tripped across this one as I inadvertently came into this forum! Try using creatine. This is a naturally occurring substance in your body, produced by your liver (I think) which a lot of athletes use as an aid for training. One of the effects is that it helps with water retention in the muscles so that your body stores more water, particularly if you are using a gym. I use it for "ballast" in the summer when I'm sailing and also find it useful for training.
Don't worry, it's not an anabolic steriod or anything like that!
Do a search for "creatine" on Google or ask your doctor about it. In the UK, it's sold in health shops and is perfectly legit.