The Improved Multi Select module for Drupal 7 replace the default multi-select boxes with two container list and search. The first container holds the options that are available, and the second container holds the options that have been selected. The two containers are separated with add and remove icons. You can select an item from the first container by click the add button and add it to the second container. Likewise you can select an item from the second container. click the remove button, and it goes back into the unselected container. You can also search by available options using search field.
Install and configure (admin/config/user-interface/improved_multi_select)