Multi select lists can be a pain for end users.
Issue 1 - how do you inform the user that they need to hold the Ctrl/Cmd key down to select multiple items?
Issue 2 - with large lists, how do we provide a visual cue of what the user has selected?
Solution 1
Use the excellent jQuery tipsy plugin to show a tooltip when they hover over your select list:
Solution 2
Display the selected items next to the select list.
