For me it is not over training as TC says.
Recovery, sleep, nutrition and hydration.
I make sure to get a lot of potassium, sodium, calcium, magnesium, manganese and zinc to help my body hold fluids. Even eating a really clean whole foods diet I do not get enough vitamins and minerals so I supplement them. I like to use FitDay to see what amount of nutrients I am getting from my foods so I can see exactly what I need to add supplement wise.
I never used any OTC type supps, I think they are all BS.