I would try going back to a spill proof sippy to see if that helps. We have these and they are the hardest to spill, and hard to get to leak by shaking, I have been able to find. They are Playtex brand.
i have no answers but max messes w/ straw cups in the car and its especially annoying when no one is in the backseat with him (Which is often.) so he gets water on himself. for the most part though he has stopped. we give him open cups now at home and he doesnt pour them out anymore. maybe shes just bored?