How to open a popup only once?

I plan to add a new popup window to my homepage. But I do not want to open it every time my visitors return to my page.

How can I do this? Any suggestions?

[MOD EDIT]Technical Issue[/MOD EDIT]

You'd have to set a cookie on the visitors computer.

or if you know a server side programming language... you can use sessions to track your visitors ... a session can make use of a cookie or a variable

vBulletin (this forum script) uses sessions as well... if you look above into the Address of the site - you'll see the:

s=(some mumbo jumbo random alphanumeric chars)

thats the session id that vBulletin passes to keep track of your being 'logged' in to the forums :)