We say “shake before enjoying”, and the best way to give it a good shaking is before the carton is opened for the very first time in your kitchen. After that, the best way to stir up the contents, whether cream on the top or cocoa on the bottom, is by pinching the top closed at its center and swirl the container so that the bottom swings out in a circular motion while the top stays as stationary as possible. This technique should keep the milk from splashing out the opened top - if you vigorously shake the milk in an up-down motion spillage is very difficult to contain under any scenario.  For people who do prefer to more vigorously shake our milk each time it is used, our suggestion would be to simply pour it into a separate reusable container with a re-sealable lid.  Here is an example of a generic half-gallon glass container commonly available at large grocery and houseware stores at a reasonable price.