The Improved Multi Select module for Drupal 7 replace the default multi-select
boxes with two pannel list and search.
The first panel holds the options that are available, and the second panel
holds the options that have been selected.
The two panels are separated by add and remove buttons.
You can select an item from the first box, click the add button, and
add it to the second box. Likewise, you can select an item from the second box,
click the remove button, and it goes back into the unselected box.
You can also search by available options using search field.

Install and configure (admin/config/user-interface/improved_multi_select)