Session cookie (cookie with no expire date) or sessionStorage (HTML5). Basically, the first time a user shows up on that page, display the pop up (modals are less annoying, and can't be blocked by pop up blockers) and set the value of a session cookie or sessionStorage item to true. On every page load, check to see if it exists; if it does, do nothing, it already popped; if it does not, pop it up.
And don't use document.write or .writeln. Ancient, deprecated, and cannot be used after a page loads.
If anyone knows of a website that can offer ColdFusion help that isn't controlled by neurotic, pedantic jerks* (stackoverflow.com), please PM me with a link.
* The neurotic, pedantic jerks are not the owners; just the people who are in control of the "popularity contest".