coordination between iphone, ipad and mac

I am a consultant, obviously not on software, I am on the road and change and make appointments on the fly between iphone, now I pad or the mac.
how do i make sure the last change i made is the one that is on my calendar regardless of the input device ?
It appears that one device overrides, I have mobile me and sense that is a solution but how.
Patience would be appreciated I am getting enough grief over this