I agree, water.
I always had that problem when I used to run a lot. When I started running again not long ago, sure enough, same thing. That was what the trainer at the gym told me, and I thought he was crazy, but it really did make a difference. I also found that eating a few bananas throughout the week helps to. (I think it's the phosphorus?)
The other problem I had (and STILL have) is that after running, I would cough horribly for days on end. I assumed it was because I have such bad sinus/allergy problems, but from what I've been told by several people (which could be a load of hock for all I know) is that when you are not getting enough physical activity, you're lungs do not inflate to their max/norm capacity when you breath and they become more coated with mucus. The increase in physical activity causing them to began working towards their capacity and breaks up the mucus, resulting in the coughing. (If anyone can confirm that that is true, and not just something the trainer told me to sound like he knew what he was talking about, please do!)