Mine leaks too.Drives me crazy. Someone a while back suggested that you make a rubber washer out of inner tobe or something, and put it behind the rubber disc already on it. I don't know if it would work but sounds good.
What I do is: don't fill it all the way to the top, and when closing it, be sure to give it a little extra push. It should click more than once to be fully closed. Even then, I get a little leakage around the cap sometimes.