This script creates a cookie controlled pop up window with different expiration dates based on the user's selection.
View the source of this page for cookie code and notes. You also need the code from the pop up window for this to work.
Click here to reset the cookie, then reload the page to make the popup reappear.
There is another version of this script that allows you to add a flag to your links to further control when the pop up will appear. A common usage would be to prevent pop up until the user leaves your site (i.e. a "site exit" pop up).
You shouldn't have any trouble using this JavaScript, but if you have any questions, please post them in the this topic at JimWorld's forums.