I have problems staying hydrated as well. My body can't seem to retain water, and anything I drink gets peed out within a few hours. If it gets bad enough, I start feeling light headed and dizzy.
I've been trying to INCREASE salt intake. The more salt (and other electrolytes) in your body, the more fluid your body can hold on to, and hopefully, for me, stay hydrated better. It's why Gatorade is so popular for hydrating athletes.